A CONTRIBUTION 
TO THE HISTORY OF THE 
GERM-LAYERS IN CLEPSINE. 
BY 
C. O. WHITMAN. 
REPRINTED FROM THE JOURNAL OF MORPHOLOGY, 
VOL. I., NO. I, SEPTEMBER, 1887. 
BOSTON: 
GINN AND COMPANY. A CONTRIBUTION 
TO THE HISTORY OF THE 
GERM-LAYERS IN CLEPSINE. 
BY 
C. O. WHITMAN. 
REPRINTED FROM THE JOURNAL OF MORPHOLOGY, 
VOL. I., NO. I, SEPTEMBER, 1887. 
BOSTON: 
GINN AND COMPANY. A CONTRIBUTION TO THE HISTORY OF THE 
GERM-LAYERS IN CLEPSINE. 
C. O. WHITMAN. 
CONTENTS. 
PAGE 
I. Cleavage and the early estab- 
lishment of Bilaterally Sym- 
metrical Relations 108 
1. First Period, ending with the 
Eight-cell Stage 108 
2. Second Period, ending with 
the formation of Proliferat- 
ing Blastomeres (Telo- 
blasts) hi 
II. Origin of the Mesenteron .... 113 
1. Historical and comparative . 113 
Clepsine 113 
Nephelis 116 
Rhynchelmis (Euaxes) ... 124 
Branchiobdella 130 
2. Observations 133 
Recapitulation 137 
III. The Ectoderm and its Products, 139 
1. The Ectoderm 139 
2. The Ventral Nerve-chain... 139 
Use of the term Germ- 
bands 140 
Germ-bands of the Head, 140 
Germ-bands of the Trunk, 142 
INTRODUCTORY. 
PAGE 
Origin of the Ventral 
Nerve-chain 142 
(a) Historical and Critical, 142 
(3) Observations .. 155 
3. The Larval Gland-cells .... 157 
4. Primary Sense-organs of the 
LiP 159 
5. The Nephridia 159 
6. The Pharyngeal Clefts 162 
IV. Special and General Questions, 162 
1. Larval Nephridia 162 
2. Significance of Nephric Cell- 
cords 165 
3. Questions relating to the 
Nephridia 166 
Original Function 166 
Original Basis 167 
4. The Origin of the Epider- 
mis 169 
5. Significance of the Telo- 
blasts.. 171 
6. Foetal and Larval Types of 
Development 172 
THE origin and fate of the germ-layers in Clepsine have been 
the subject of investigation and critical discussion by a number 
of recent writers, who have reached conclusions so widely at 
variance that one might still say with Balfour, — “Our knowledge 
of the development of the Discophora is in a very unsatisfac- 
tory state.” The origin of the mesenteron is very difficult to 
trace, and ignorance of its history has led to the greatest con- WHITMAN\ 
[Vol. I. 
fusion in the identification of the germ-layers, and to most 
contradictory interpretations of the strata composing the germ- 
bands. The association of neuroblastic with mesoblastic ele- 
ments in the germ-bands, as maintained in my first paper on 
this subject, proved a serious stumbling-block to Balfour. 
The germ-bands in a closely related group of annelids, the 
Oligochmta, were held to be purely mesoblastic, and my sug- 
gestion that they contained a neural stratum appeared to 
stand in plain contradiction with well-established views as to 
the origin of the nervous system. Moreover, Kowalevsky, 
whose brilliant success in extending the germ-layer theory to 
the invertebrates had rendered his authority preeminent, had 
stated, as a fact settled by his own observations, that the ner- 
vous system of Clepsine, like that of Lumbricus and other 
oligochaetous annelids, was derived from the “upper layer,” i.e., 
the epidermal layer. This statement, although based upon an 
evidently hasty and unreliable examination of a few poorly pre- 
served eggs of Clepsine, was corroborated by more extended 
studies on Rhynchelmis (Euaxes) and Lumbricus, and later, 
by the researches of Hatschek, Kleinenberg, and others. The 
more trustworthy statements of Metschnikoff, published in the 
same year with Kowalevsky’s “ Embryological Studies on 
Worms and Arthropods,” escaped the attention of Balfour, and 
thus the testimony of numerous excellent observers as well as 
theoretical considerations appeared to stand in the way of ac- 
cepting my conclusions. The contradictory results since reached 
by Bergh and Nusbaum have only made it still more desirable to 
reexamine the subject with greater care and thoroughness. 
Bergh’s important researches on the development of the 
Gnathobdellidse have led him to dispute the concurrent testi- 
mony of previous investigators on the origin of the epidermal 
layer in Clepsine; and thus we are left in a state of uncertainty 
regarding the origin and limitations of the germ-layers, not very 
far removed from that in which Hoffmann found himself, when, 
at the end of his second memoir on this subject, he frankly con- 
fessed his inability to distinguish “ Keimblatter ” in the Hiru- 
dinea. 
Respecting the histological differentiation of the germ-band 
strata, there is still less unanimity of opinion. Even the latest 
writers, Bergh and Nusbaum, have failed to agree on the No. 1.] 
GERM-LAYERS IN CLEPSINE. 
107 
derivation of the ventral nerve-cord, and my studies have led 
me to results which contradict the conclusions of both these 
authors. Although the origin and development of the neph- 
ridia and the sexual organs have received special attention in 
recent papers, these questions are still very far from being 
definitely settled. The precise origin of the nephridia forms 
just now a question of considerable theoretical importance,— 
an importance which will be found to be very much increased 
by the facts to be presented in this paper, and by the parallel 
results reached by Wilson in his study of Lumbricus. We have 
long been prepared to believe in the homology of the germ- 
bands of the Hirudinea and the Oligochaeta; but such a com- 
plete parallel, both in the mode of origin and in the histological 
development, as is shown in these two papers, will certainly be 
a surprise to most embryologists. The observations have been 
made, independently, on representatives of two different groups 
of annelids, and they confirm each other in a manner too posi- 
tive to leave any room for a reasonable doubt of their essential 
accuracy. Allowing that they are correct, it will be seen that 
they furnish what we have long stood in need of, — a satisfac- 
tory basis for the comparative study of the germ-layers in 
annelids, — and that they give us one more clue to the ancestral 
history of the vertebrates. 
For the observations recorded in this paper, I had only a 
single batch of eggs of Clepsine parasitica (?) Say, and a few 
eggs of C. complanata obtained at Naples. This scanty 
material did not enable me to carry the investigation beyond 
the stage in which the concrescence of the germ-bands is nearly 
completed. I am now able, however, to account for the origin 
of all the germ-layers, and to give the earlier history of the 
germ-bands. I have been able to add something to our knowl- 
edge of the development of the head, and I hope soon to 
obtain material for a thorough study of this very important part 
of the subject. The methods employed have been described 
in the “American Naturalist,” Nov., 1885, pp. 1134-1135; and a 
preliminary notice of the results obtained was published in the 
“ Zoologischer Anzeiger,” No. 218, 1886. 108 
WHITMAN. 
[VOL. I- 
I. CLEAVAGE, AND THE EARLY ESTABLISHMENT 
OF BILATERALLY SYMMETRICAL RELATIONS. 
An accurate acquaintance with the principal events of cleav- 
age is indispensable to a clear understanding of the derivation 
of the germ-layers; for these layers, and even the more impor- 
tant organs arising from them, can be traced directly to special 
blastomeres. 
First Period, ending with the Eight-cell Stage.—A sketch 
of the cleavage, based upon results published in an earlier paper 
(No. 1, pp. 49-58, 75-76) is here introduced for the sake 
of clearness in the discussion and descriptions which are to 
follow. 
The first two cleavage-planes are vertical, dividing the egg into 
four macromeres, three of which are relatively small, but nearly 
equal. The fourth, larger segment, which contains the remnants 
of the polar rings characteristic of this egg, occupies a posterior 
position with reference to the future embryo, while the anterior 
and the two lateral positions are held by the three smaller macro- 
meres. The next step consists in the formation of four ectoblas- 
tic micromeres, which eventually present the figure of a quarter- 
foil at the animal pole. The posterior macromere (Diag. 1, x) 
first buds off the ectoblast xx, then the right macromere, by 
gives rise to the second ectoblast, bb, and immediately after- 
ward the third and fourth ectoblasts, aa, and cc, are produced 
simultaneously by the left macromere, a, and the anterior macro- 
mere c. We arrive thus at the eight-cell stage so common 
among worms and molluscs, represented by four macromeres 
surmounted by four micromeres that lie in cruciform order 
in the boundary-lines of the mother-cells. 
The complete orientation of this stage embraces some rela- 
tions of position yet to be noticed. The alternation of the 
micromeres with the macromeres is an arrangement brought 
about by a rotation of the whole quarterfoil, each segment having 
(6.) Embryology of Clepsine. Quart Journ. Mic. Sci., XVIII., 1878. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
109 
Semi-diagrammatic surface views of the egg of Clef sine in different stages 
of Cleavage. 
Diag. i. — The eight-cell stage, showing the relation of the embryonic axis 
to the first two cleavage-planes. The arrow, 2-2, shows the median plane of the 
embryo, and the four small arrows indicate the direction in which the four 
micromeres have rotated on the axis of the egg. 
Diag. 2. — The posterior macromere (Diag. 1, x) has just divided into the 
neuro-nephroblast x', and the two mesoblasts x and xy. 
Diag. 3. — Same stage seen from the lower pole. 
Diag. 4. — Stage in which all the teloblasts and the blastodisc have been 
formed. 
I-I and 1-1.— First cleavage-plane; II—II and 2-2, second cleavage plane; 
a, left macromere; c, median and anterior macromere; b, right macromere; 
x, posterior macromere; aa, cc, bb, xx, micromeres derived from a, c, b, and 
x', neuro-nephroblast; xy, left mesoblast; x (Diag. 3.), right mesoblast; nb, 
neuroblast; nfh, nephroblast; l, lateral (median in this stage) teloblast. WHITMAN. 
[Vol. I. 
moved through an arc of 450 from its place of origin. Had 
the segments maintained their original positions, it is evident 
that the arrangement would have been opposite, instead of alter- 
nate. The direction of rotation,1 which is the same as that fol- 
lowed by the hands of a watch, is determined by the mode of 
origin of the segments. The segment bb, arising subsequently 
to the segment xx, pushes the latter to the left, and the posi- 
tions thus established for the first two segments predetermine 
corresponding positions for the two last-formed segments (aa and 
cc). Such a point might at first sight appear too trivial for special 
notice, but its importance becomes apparent in view of a ques- 
tion, now attracting a good deal of attention, as to the relation 
of the first two vertical cleavage-planes to the median plane of 
the embryo. There is no danger of being too exact in regard 
to such relations, and the egg of Clepsine furnishes a case in 
which they can be determined with a certainty that precludes 
all controversy. As the micromeres represented, originally, the 
upper angles of the macromeres, it is evident that the planes of 
their apposed faces, marked by the boundary lines 1 and 2, must 
be regarded as parts of the cleavage-planes I and II, although, 
in consequence of the rotation of the quarterfoil, the former now 
form angles of about 450 with the latter. The boundary lines, 
I and 2, present the form of a cross, the arms of which, if ex- 
tended, would bisect each of the macromeres. That limb (2) 
of the cross, which originally formed a part of the cleavage-line 
II, now marks the median plane of the future embryo. Here 
then is a case in which the median plane of the embryo is very 
clearly defined, and in which it coincides with neither of the first 
two cleavage-planes, but forms an angle of 450 with each of 
them. 
The cleavages resulting in the eight-cell stage are comparable 
with the first two meridional cleavages and the first equatorial 
or horizontal cleavage in the eggs of amphibia and some 
fishes. 
The further history of the quarterfoil of micromeres has not 
been worked out with sufficient thoroughness to say positively 
and precisely what part they play in forming the embryo; but 
it seems quite certain that they contribute to the formation of 
1A similar rotation has been described by Hatschek (No. 35, p. 6) in Eupomatus. No. i.] 
GERM-LAYERS IN CLEPSINE. 
111 
the epidermal stratum of the ectoderm, and it is possible that they 
are directly employed in the formation of more important parts 
of the head. Smaller micromeres are gradually added round 
the primary quarterfoil, in the proliferation of which each of the 
macromeres appears to share. A disc of small cells is thus 
formed in which it is impossible to trace the genetic history of 
individual elements. 
Second Period, ending with the Formation of Proliferating 
Blastomeres. —All regularity of cleavage ends with the eight- 
cell stage. Henceforth several distinct forms of cleavage will 
be carried on simultaneously, each restricted to special areas or 
blastomeres. Although there is scarcely anything in the exter- 
nal appearance of the eight-cell stage to indicate the relation of 
its parts to the future embryo, yet we know by what follows that 
an immense work has already been accomplished. All those 
fundamental conditions and relations implied in the terms ante- 
rior and posterior, right and left, dorsal and ventral, are now 
definitely established. The ground-plan of the future structure 
is there, and the segregation and distribution of the building 
material have advanced far toward completion. 
The second period concludes the finishing strokes of cleavage, 
carrying us from the eight-cell stage to that in which the pro- 
liferation of the germ-bands begins. The prospective character 
of the work becomes more and more manifest, and architectural 
forecasts begin to reveal themselves. It is a period of preparation 
in which everything is ordered and appointed for the formative 
work with which the third or embryonic period begins. Proliferat- 
ing blastomeres are created, grouped, and stationed according to 
the special kinds of work which they, as the artisans of the third 
period, are destined to accomplish. There are exactly thirteen 
of these blastomeres, symmetrically arranged in three primary 
groups. One group, consisting of three entoblasts, is represented 
by the macromeres, a, b, and c, or rather, will be represented by 
them at the end of this period, after they have ceased to con- 
tribute to the ectoblastic disc. The second and third groups, 
consisting respectively of two mesoblasts and eight ectoblasts, are 
yet to be developed from the large posterior macromere, x. 
The macromeres a, b, and c take no further part in the cleavage, 
if we except the budding off of ectoblastic micromeres at the 
animal pole, which does not sensibly diminish their size or alter 112 
WHITMAN. 
rvoL. 1 
their general appearance. Throughout the embryonic period, 
until long after hatching, these huge segments preserve their 
individuality, only undergoing such slight changes in form and 
position as are induced by the development of the germ-bands 
and the epibolic expansion of the ectoderm. They contain most 
of the food-yolk, which is utilized in the long post-embryonic 
pseudo-larval period, and the elements which are to form the 
mesenteron. 
The posterior macromere, x, on the other hand, undergoes 
sucessive cleavages, resulting in the production of ten blastomeres, 
or teloblasts, arranged in two bilaterally symmetrical groups, at 
the posterior edge of the blastodisc (Diag. 4). 
The first cleavage-plane runs obliquely, beginning a little to the 
left of the upper angle and taking a slanting direction towards the 
right side, thus cutting off (x, Diag. 2) about one-third of the origi- 
nal macromere. Then follows a second cleavage, at right angles to 
the first, cutting the larger segment into nearly equal parts (x and 
xy, Diag. 3 ). The posterior macromere is now represented by three 
sub-equal segments: one of these, x', which may be called the 
neuro-nephroblast (uprimary neuroblast,” in my first paper), lies 
at the posterior edge of the blastodisc, more on the right than the 
left side. The second and third, representing the mesoblasts, are 
also asymmetrically placed, one (x) occupying a central posi- 
tion at the lower pole of the egg (Diag. 3), the other (x y) 
lying behind it, and to the left of the neuro-nephroblast (x'). 
Neither in the general appearance nor in the relative positions of 
the two mesoblasts is there anything indicative of their homo- 
typical character. They appear to be vitelline spheres of the 
same nature as the three macromeres, a, b, and c, and have 
always been so regarded by earlier observers. In the course of 
this period a shifting of position among the cleavage-spheres 
takes place, which brings the mesoblasts into harmony with the 
bilateral symmetry of the egg. The three macromeres lengthen 
backward, slowly flowing over the mesoblasts and more or less 
completely enveloping them, so that one or both of them may 
entirely disappear from view, or, at least, become so obscure in 
outline as not to be easily recognized. In its backward elonga- 
tion the anterior macromere c takes up a median ventral posi- 
tion between a and b, and usually carries the mesoblast x 
towards the hind end at the same time that it incloses it. No. I.] 
GERM-LAYERS IN CLEPSINE. 
113 
The right mesoblast x thus becomes imbedded mainly in 
c, but usually lies partly in b. Meanwhile, the mesoblast, xy, 
takes up an opposite position in the left macromere, a. I have 
never seen a single case in which bilateral symmetry was com- 
plete with respect to the mesoblasts; but the final position is 
always such that each proliferates exclusively for the germ- 
band of its own side. Their relative positions in transverse 
section are shown in Fig. 2, PI. IV. In Fig. 3, representing 
a sagittal section of C. complanatat the right mesoblast, is 
placed much farther forward than is usually the case in C. 
marginata. 
By successive vertical cleavages the neuro-nephroblast is 
split up into eight octoblasts, arranged symmetrically in two 
groups at the posterior edge of the blastodisc, as shown in 
Diag. 4. These cells have a superficial position at first, but 
the two median ones are very soon covered by the expanding 
blastodisc, and the rest are later overgrown by the same 
elements. 
Towards the close of this period, when all arrangements for 
the formation of the embryo have been completed, free nuclei 
begin to appear in the surface of the three entoblasts, a, b, c. 
The origin and fate of these “ entoblasts ” have been traced 
with considerable care, and the results of my study on this 
point have confirmed the opinion that they give rise to the 
mesenteron. 
II. ORIGIN OF THE MESENTERON. 
1. Historical and Comparative. 
Clepsine. — Grube (No. 2), and Leuckart and Rathke (No. 3), 
derived the entoderm by delamination from the blastoderm. 
But these authors, whose observations on this subject were made 
long before the introduction of the microtome, were not able to 
bring any direct evidence of such a mode of origin. It was 
simply the most rational conclusion open to them, considering 
the methods at their command, and what was then known 
(2.) Grube, A. E. Untersuchungen iiber die Entwicklung der Clepsinen. Konigs- 
berg. 1884. 
(3.) Rathke, H., and Leuckart, R. Beitrage zur Entwicklungsgeschichte der 
Hirudineen. Leipzig. 1862. 114 
WHITMAN. 
[Vol. I. 
respecting the genetic relations of the germ-layers. Even 
Hoffmann (No. 4, p. 45), working as late as 1877, with “Pikro- 
carmin gefarbten Querschnitten,” arrives at the same conclusion, 
and expresses it in words that appear to be, in great part, a 
direct transcript of Leuckart’s phraseology. 
According to Robin (No. 5, pp. 297-298), the entoderm 
arises as a solid cord of cells in the axis of the mass rep- 
resented by the three entoblasts, a, b, and c. A lumen 
first appears in the oesophageal portion of the axial cord, 
and is gradually extended backward, thus forming a digestive 
tube, with all the yolk lying external to it. Somewhat later 
(5-8 days after exclusion) the three entoblasts (“globes 
vitellins”) undergo cleavage, breaking up into large cells, that 
form “ la couche moyenne de I'intestin et particularement la 
couche hepatiqneB 
In my memoir on “ the Embryology of Clepsine ” I was 
unable to give a complete history of the origin of the mesen- 
teron from the three entoblasts, but the facts there presented 
appeared to leave little room for doubt. Both my observations 
and conclusions have been contradicted by one of the latest 
writers on the subject. Soon after the publication of my paper 
came Hoffmann’s (No. 6) second contribution to the embryology 
of the Hirudinea, in which he declares his opposition to my 
views in the following words: “ Ich muss dieser Entstehungs- 
weise des Darmepithels auf das bestimmteste wiedersprechen. 
Die ‘ dark spots in the opaque yolk,’ welche nach Whitman in 
der Zeit, dass die Keimstrange sich ausbilden, so recht deutlich 
auftreten, sind, ich vviederhole es, nichts anderes als Protoplasma- 
Masse, welche sich aus dem Deutoplasma neu gebildet hat, sich 
abschniirt und so an der Bildung der Keimstrange sich 
betheiligt. Bei ausgeschliipften Embryonen jindet man inner- 
halb des Dotters auch nicht einen Kern. Waren sie vorhanden, 
so miissten sie an in Pikro-carmin gefarbten Schnitten sichtbar 
werden, denn die intensiv rothe Farbe der Kerne und des Proto- 
(4.) Hoffmann, C. K. Zur Entwickelungsgeschichte der Clepsinen. Niederland- 
isches Archiv fiir Zoologie, IV., H. I, p. 31. 1887. 
(5.) Robin, C. Memoire sur le Developpement Embryogenique des Hirudinees. 
Paris. 1875. 
(6.) Hoffmann, C. K. Untersuchungen iiber den Bau und die Entwickelungsge- 
schichte der Hirudineen. Haarlem. 1880. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
115 
plasmas, wie die grime der Dotterkornchen ist zu charakteris- 
tisch, als dass man dieselbe ubersehen konnte.” (p. 53). 
“ Die Muskelfaserschicht und das Epithelium des Chylus- 
darmes entstehen beide aus urspriinglich von dem Keimstreifen 
herriihrenden zelligen Elementen ” (p. 51). The same origin 
is claimed for the intestine (Enddarm) and the oesophagus 
(Schlunddarm). 
Metschnikoff (No. 7, p. 672) expresses no decided opinion 
on the origin of the mesenteron, but throws out the following 
suggestion. Speaking of the inner stratum (mesoderm) of the 
germ-bands, he says: “ Man kann sehr leicht die Uberzeugung 
gewinnen, dass das sich spaltende Blatt die iiussere (vielleicht 
auch die innere') Wand des Mitteldarmes, den sog. Fettkorper, 
und die Segmentalorgane liefert.” 
The latest authority on this subject is Joseph Nusbaum (No. 
8). In the introduction to his paper, Nusbaum states at some 
length Hoffmann’s erroneous views concerning the nature of the 
“ entoplasts,” and then announces, as a discovery of his own, 
the fact that these entoplasts (“ Hotsproloplasmiques”') represent 
the entoderm. Why so much attention should be lavished on 
Hoffmann’s ideas, which had already been refuted by Bergh, 
and why the fact should be suppressed that the origin of the en- 
toderm had been ascertained eight years before, I will leave 
Nusbaum to explain. To discover what has already been dis- 
covered, and refute what has already been refuted, is a double- 
headed offence, inexcusable if the result of ignorance, unpar- 
donable if done deliberately. Nusbaum very narrowly escapes 
from this charge, for later on he feels constrained to drop the 
following saving remark: “ Selon Whitman, l’epithelium intes- 
tinal se forme aux depens de grands globes entodermiques, a la 
surface desquels se montrent les noyaux libres et ensuite une 
couche de cellules epitheliales qui limite de tous les cdtes le 
vitellus nutritif. En un mot, d’apres Whitman, l’entoderme 
secondaire se forme de l’entoderme primitif.” 
Nusbaum carefully refrains from any direct acknowledgment 
(7.) Metschnikoff, Elias. Beitrage zur Entwickelungesgeschichte einiger 
neideren Thiere. Vorlaufige Mittheilung. Melanges Biologiques VII., p. 671. 1871. 
(8.) Nusbaum, Joseph. Recherches sur l’organogenfese des Hirudinees (Clepsine 
complanata Sav.). Archives slaves de Biologic, /. Base. 2, pp. 310-340, and Base. 
3, pp. 539-556. 1886. Published separate. WHITMAN. 
[Vol. I 
that his conclusions had been anticipated, and turns at once to the 
more agreeable task of contradicting Hoffmann’s account. His 
own observations are then detailed as follows. Beginning with an 
embryo in which the somites are already distinctly marked from 
end to end, and the nephridia differentiated, he says: “II se 
montre sur toute la surface externe du vitellus une couche pro- 
toplasmique granuleuse (en) avec des noyaux ovoides et arron- 
dis; cette couche, developpee aux depens des elements cellulaires 
de l’entoderme primitif, se transforme ensuite en epithelium intes- 
tinal. Hoffmann a observe aussi une semblable couche proto- 
plasmique avec des noyaux ovoides, mais il l’a consideree comme 
un produit du mesoderme. Cependant j’ai vu plusieurs fois, 
sur des coupes tres minces et parfaitement colorees, que cette 
couche est delimitee d’une maniere tres nette de la couche de 
cellules plates du feuillet visceral du m6soderme, tandis que du 
c6te interne la couche protoplasmique passe directement dans 
le vitellus; ainsi l’origine entodermique de cette couche ne peut 
6tre soumise a aucun doute ” (p. 9). 
This is all that Nusbaum has to tell us about the origin and 
history of the entoplasts. The whole alimentary tract is lined 
with cells of the same origin. “Ainsi le resultat final est que 
l’dpithelium (plat) tapissant la cavite de la trompe reprdsente 
un produit entodermique. Tout l’intestin represente 
de m6me un produit entodermique” (p. 11). The sequel will 
show that Nusbaum has added very little to what was already 
known on this subject. 
Bergh (No. 9, pp. 259-260) confirms my statements in regard 
to the existence of peripheral nuclei in the three entoblasts, but 
rejects the hypothesis that they represent the entoderm, as in- 
compatible with what is known about the origin of this layer in 
the Gnathobdellidae. 
Nephelis.—According to the observations of Kowalevsky 
(No. 10, p. 3), Butschli (No. 11, ), and Bergh (No. 9, p. 259), 
the entoderm in Nephelis arises immediately after the eight-cell 
(9.) Bergh, R. S. Die Metamorphose von Aulastoma gulo. Arbeiten a. d. 
zool.-zoot. Institut in Wurzburg. VII., H. 3, p. 231. 1885. 
(10.) Kowalevsky, A. Embryologische Studien an Wiinnern und Arthro- 
poden. Mem. dePAcad. Imp. de Sc. St. Petersburg. XVI., No. 12. 1871. 
(11.) Butschli, O. Entwicklungsgeschichtliche Beitrage. Zeitschr. f. wiss. 
Zool. XXIX., p.239. 1877. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
117 
stage is reached, consisting at first of a few small cells lodged 
between the micromeres and the macromeres, and derived in all 
probability from the latter. Bergh is inclined to the belief that 
this mode of origin holds true of the Rhynchobdellidae as well as 
the Gnathobdellidae. He finds in the egg of Clepsine certain 
cells beneath the blastodisc, in a position which corresponds 
perfectly to that of the first entoderm cells in Nephelis, and 
hence infers that they represent the entoderm of Clepsine, 
although he has not traced their development into this layer. 
Notwithstanding the remarkable difference in the position of 
the “ residual ” yolk, which distinguishes the two types of leeches 
represented by Clepsine and Nephelis, it must be taken for 
granted, so long as no positive proof to the contrary has been 
produced, that the mesenteron arises in essentially the same 
manner in both cases. I have ascertained enough of the his- 
tory of the cells in Clepsine, which Bergh identifies with the 
entoderm of Nephelis, to convince me that he is partly right on 
this point; but it is an error to suppose that they constitute the 
whole, or even the larger part, of the entoderm in Clepsine, and 
the observations of Kowalevsky and Biitschli fall very far short 
of establishing such a conclusion in the case of Nephelis. 
Kowalevsky gives no figures, and offers only the following brief 
sketch of the germ-layers, which he illustrates by referring to 
Rathke’s plates: — 
“ Was die Scheidung der Blatter anbetrifft, so ist dieselbe 
schon auf der Fig. 13, Taf. I., bei Rathke zu entdecken; die 
oberen Zellen, jj\ bilden das obere oder sensorielle Blatt, die 
zwischen denselben und den grossen Furchungskugeln lieg- 
enden — das Darmdrusenblatt (Figs. 14 and 15), und die drei 
grossen Kugeln — das mittlere Blatt. Weiter umwachsen die 
Zellen des oberen Blattes die ganze Masse von alien Seiten; 
die Darmdriisenblattzellen wachsen sehr schnell, verlieren ihr 
korniges Aussehen, werden zu grossen hellen Zellen und 
drangen dabei die grossen Zellen des mittleren Blattes nach 
hinten, welche durch Abschniirung zwei Zellenreihen bilden, 
die bekannten Keimstreifen der Nephelis sind.” (No. 8, p. 3.) 
Kowalevsky’s interpretation of the three large segments — 
the homologues of the entoblasts in Clepsine—as mesoblasts, 
has been corrected by Biitschli, and his statement on the origin 
of the entoderm is as indefinite as it is brief. 118 
WHITMAN. 
[Vol. I 
So far as the published records show, Biitschli is the only 
one who has thus far undertaken to trace the origin of the 
entoderm in Nephelis with that precision and attention to 
details which are now required in such work. But there are 
some important gaps in his work which must be filled before 
the origin and relation of the germ-layers can be satisfactorily 
determined. The posterior macromere in Clepsine, as we 
have seen, is the sole source of the germ-bands, and its history 
is the key to subsequent development. Unfortunately our 
knowledge of this macromere in Nephelis is not complete 
enough to warrant the assertion that it plays the same rdle; 
but the facts, so far as they go, point in this direction. When 
we remember that the germ-bands of Nephelis have been 
traced by Bergh (No. xo) to ten terminal cells, which we 
have every reason to suppose are identical with the ten telo- 
blasts of Clepsine, and further, that the eight-cell stage arises 
in the same manner in both cases, it seems incredible that there 
should be any radical differences in respect to the fate of the 
posterior macromere. If, however, this macromere in Nephelis 
is converted into the ten teloblasts of the germ-bands,— and there 
is nothing against, and everything for, such a supposition, — it 
is plain that Biitschli’s observations on the origin of the germ- 
layers are very far from complete. It is simply incredible, in 
view of what happens in Clepsine, that the whole entoderm and 
mesoderm should arise in the manner described by him. If the 
deep cells discovered by Biitschli represent only the earliest 
and most anterior portion of the entoderm, then the chief diffi- 
culty in the way of reconciling the two types of development 
disappears. This appears to be the only mode of reconciliation 
open to us, if my observations on Clepsine are correct; and it 
is entirely permissible to hold this ground until some one has 
cleared up the history of the posterior macromere in Nephelis. 
Robin (No. 5, p. 146) attempted to do this, but his methods 
of study were not equal to the task, and he fell into the error 
of supposing that the products of this macromere represented 
the dorsal moiety of the ectoderm. Biitschli has added but 
little on this important point; but a summary of his studies on 
the germ-layers will be needed in order to place the subject in 
a clear light. 
Biitschli’s account begins with the eight-cell stage, with what No. 1.] 
GERM-LAYERS IN CLEPSINE. 
119 
I have called the second period in Clepsine. This period is 
initiated by two events,— (1) the cleavage of the posterior 
macromere, and (2) by the appearance of two small cells 
beneath the micromeres. Biitschli directs his attention almost 
wholly to the second of these events, and, with the exception 
of a few incidental remarks, completely ignores the first. 
The posterior macromere first divides into two equal parts, 
one of which occupies nearly a central position on the lower 
face of the egg, while the other lies behind and slightly above, 
abutting against the posterior edge of the ectodermic quarter- 
foil. It will be remembered that similar relations are constant in 
the corresponding stage of Clepsine, with the single difference 
that the posterior of the two segments (the neuro-nephro- 
blast) is usually only about one-half the size of the other. 
It is a fact of considerable importance to us that the second 
period is introduced in both cases in the same remarkable 
manner (i.e., by the cleavage of one macromere), for we are 
thus assured that events are still moving on in parallel courses. 
I am quite confident that the two segments resulting from the 
division of the posterior macromere will be found to correspond 
to the neuro-nephroblast and the primary mesoblast of Clepsine, 
and I shall henceforth designate them by these names. One 
fact only, noticed by Biitschli, throws a little doubt on the 
identification of the neuro-nephroblast. This segment buds 
off two micromeres behind the four already formed at the 
animal pole, and the six micromeres of this stage are arranged 
in two longitudinal rows.1 I have seen nothing in the egg of 
Clepsine comparable with the last two micromeres, but I have 
shown (No. 1, p. 54, x4) that two ectoblastic micromeres arise 
in a little later stage, not directly from the neuro-nephroblast, 
but from the immediate products of this segment. 
The neuro-nephroblast next divides by a sagittal cleavage 
into two equal parts, which are probably the equivalents of the 
cells marked x* (Fig. 31, PL XII.) in my first paper; and 
then follows the cleavage of the primary mesoblast into two 
equal parts, bilaterally disposed, and corresponding in position 
and origin to the two mesoblasts of Clepsine. According to 
Robin (No. 5, p. 150) the primary mesoblast divides before 
the neuro-nephroblast, in the same order as in Clepsine. 
1 Precisely like the “ mesomeres ” in Rhynchelmis. 120 
WHITMAN. 
[Vol. I 
Possibly the order in Nephelis is variable, in which case 
Biitschli’s observations on this point would appear supple- 
mentary, rather than contradictory, to those of Robin. 
Butschli failed to trace the cleavage in detail beyond the 
sixteen-cell stage, in which (Fig. 5, PI. XVIII.), as I have 
shown, every group of segments, and nearly every individual 
segment even, can be identified with those of the correspond- 
ing stage in Clepsine. The close correspondence in parts, 
relative size, position, and axial orientation, is shown in Diags. 
5 and 6. The bulk of the egg is formed of the three macro- 
Two views of the sixteen-cell stage of Nephelis. Three deep cells, representing the 
first entoderm cells, are not shown. 
Diag. 5. — Seen as a transparent object from the upper side. (After 
Butschli.) 
Diag. 6. — Side view of the same stage. (Constructed after Butschli and 
Robin.) 
a, left macromere; c, median and anterior macromere; b, right macromere; 
aa, cc, xx, bb, micromeres derived from a, c, b, and x. 
x'x' and x'x' micromeres derived from the neuro-nephroblast (a/), which is 
now represented by two cells, x' and x>. 
xy, left mesoblast; x, right mesoblast. 
meres, one (c) of which has an anterior median position, while 
the second and third (a and b) are laterally placed. The six 
micromeres are arranged in two bilaterally symmetrical rows, 
extending from the upper angle of the anterior macromere to 
the hind edge of the egg. The two mesoblasts (x and xy) are No. 1.] 
GERM-LAYERS IN CLEPSINE. 
121 
symmetrically placed near the middle of the lower face of 
the egg. Behind and somewhat above them come the two 
daughter-cells of the neuro-nephroblast. The remaining 
three cells (not shown) lie beneath the surface, between the two 
anterior micromeres and the two mesoblasts, and represent the 
earliest portion of the entoderm. The embryonic axis has 
precisely the same relations to the primary-cleavage planes as in 
Clepsine. 
A further confirmation of the opinion that all the essential 
details of the second period in the egg of Clepsine repeat 
themselves in the egg of Nephelis is found in the peculiar 
shifting of position among the cleavage-products. Butschli 
(No. 11, p. 242) briefly alludes to the fact that the smaller 
cells become more or less completely imbedded in the macro- 
meres, and a glance at his figures shows that the anterior 
macromere passes backward between the lateral macromeres, 
and, ultimately, takes a position at the hind end of the embryo. 
The same movement takes place in Clepsine, only it is not 
carried quite so far, leaving the macromere in a median ventral 
position, stretching from end to end. 
Butschli does not discuss the nature of the cells derived from 
the posterior macromere, and only alludes to them in one place 
(p. 242) as ectoderm cells. Abandoning the only clue that could 
guide him safely through subsequent phases, his statements 
become indefinite, if not obscure; and neither his descriptions 
nor his figures are free from confusion. Beyond the sixteen-cell 
stage he is not even able to say whether his figures represent 
the upper or lower side of the egg, or to state definitely which 
side corresponds to the ventral or dorsal aspect of the future 
embryo. Entoderm cells are first represented in red, then in 
blue, and each of the three germ-layers appears in turn in red. 
Such incongruity in diagrammatic representation can have but 
one explanation. Allowing that it was the best that could be 
done under the circumstances, it is obvious enough that Butschli 
failed to leave the subject of the germ-layers in a satisfactory 
state, and that much remains to be done before the origin of 
these layers is as clear in Nephelis as in Clepsine. 
The three deep cells, which are omitted in the diagrams, were 
discovered by Butschli, and interpreted as the “ Anlage des 
Entoderms.” Their origin was not directly observed, but their 122 
WHITMAN: 
[Vol. I. 
position was held to be sufficient evidence of derivation from 
the three macromeres. The number of these cells is very soon 
raised to six, but whether by renewed proliferation on the part 
of the macromeres, or by subdivision among themselves, we 
are not informed. A little later the still more numerous ento- 
derm cells present the form of a solid axial cord, on each side 
of which is seen a row of three mesoderm cells. These two 
rows of cells are regarded as the basis of the germ-bands 
(“Keimstreifen”), but the important question of their origin 
is left undecided. The mesoderm cells increase rapidly, but 
not by proliferation on the part of the vitelline macromeres, as 
supposed by Kowalevsky. Biitschli expressly states that he 
has never seen these macromeres in process of division, until 
in a very late stage of the free embryonic life, when, as was 
first shown by Robin, they break up into a number of celis, 
which undergo resorption in the body-cavity (Bergh). 
In the next stage represented by Biitschli, a narrow, slit- 
like lumen is seen between the entoderm cells, which is the 
incipient enteric cavity. The number of entoderm cells must 
be small, as only eight are shown in optical section (Fig. 9, 
PI. XVIII.) ; and it must be noted as a still more remarkable 
fact that precisely the same number of cells appear in a much 
later stage (Fig. 12), after the formation of the oesophagus. 
They have increased immensely in size, while the three macro- 
meres have become proportionately smaller. Biitschli thinks 
the entoderm cells enlarge at the expense of the fluid food- 
material contained in the egg-case; but his figures suggest that 
the growth is at the expense of the macromeres, and analogy 
would lead one to suppose that the embryo would exhaust its 
own stock of food-stuff before drawing upon external supplies. 
It is evident that these large entoderm cells have many changes 
to undergo before assuming the form of a lining epithelium; 
but what these changes are, and how the cells multiply, are 
questions left unanswered by Biitschli’s observations. I find 
no mention of “ free nuclei,” and no indications of such bodies 
in his figures, unless the two supernumerary nuclei, shown in 
Fig. 12, may be so regarded. Such nuclei have been observed 
in the egg of Nephelis, according to Balfour (No. 12, pp. 3-9). 
(12.) Balfour, F. M. A preliminary Account of the Development of the 
Elasmobranch Fishes. Quart. Jour. Mic. Sc. XXII., p. 323. 1874. No. i.] 
GERM-LAYERS IN CLEPSINE. 
123 
“ Dr. Kleinenberg has followed a single egg through the whole 
course of its development, and concludes that the nuclei of 
Nephelis never become the nuclei of new cells.” 1 
Assuming, then, that the ten terminal cells in Nephelis are the 
homologues of the teloblasts in Clepsine, and that, accordingly, 
the primitive mesoblastic cords discovered by Biitschli arise 
neither from the micromeres nor from the three entoblastic 
macromeres, a, b} e, but from a pair of cells (x and xy) 
derived from the posterior macromere, xx, the way is clear for 
comparing the entoderm cells. The chief differences are: (1) 
the absence of “ free nuclei” in Nephelis, and (2) the contra- 
distinction in position of the three macromeres, which lie 
within the mesenteron in Clepsine, and external to it in the body 
cavity in Nephelis. The foundation of both differences is 
undoubtedly the relative abundance of food-yolk. The egg of 
Clepsine is much larger than that of Nephelis, and is completely 
filled with yolk-spherules, which are to serve as food during 
several weeks of larval life. The egg of Nephelis, on the other 
hand, has very little food-yolk, the larva depending upon the 
fluid albuminous substances contained in the egg-case. The 
abundance of yolk in the egg of Clepsine makes it necessary for 
the nuclei to seek a peripheral position, as in the case of so 
many arthropod eggs; and thus the yolk is left within the 
enteric epithelium. In the egg of Nephelis, the entoderm cells 
all escape from the macromeres at an early date, and henceforth 
they multiply by subdivision, and probably grow at the expense 
1 Balfour has here given Kleinenberg credit for what neither he nor any one else has 
ever yet accomplished. The method has not yet been discovered by which the egg of 
Nephelis can be kept alive for more than a few hours, after removal from the egg- 
case. Continuous observation on a single egg through all its stages of development 
is, therefore, a feat entirely beyond our present means. Kleinenberg’s opinion as 
to the fate of certain nuclei must rest on evidence of a much less conclusive nature 
than supposed by Balfour. It is to be hoped that Kleinenberg will yet publish the 
results of his study on Nephelis, for we certainly stand in need of more light on this 
subject. 
Bergh’s brief remark on the origin of the mesenteron may be interpreted in favor 
of the existence of free nuclei. “ Letzteres [Mitteldarmepithel] bildet sich namlich 
aus dem primaren Entoderm durch fortdauernde Kernvermehrung ; erst nach dem 
Ausschliipfen aus dem Kokon bildet es sich als eigentliches Epithel aus, indem das 
Protoplasma sich um die Kerne herum in Zellen sondert.” (No. 13, p. 294.) 
(13.) Bergh, R. S. Ueber die Metamorphose von Nephelis. Zeitschr. f wiss. 
Zool. XLI. H. 2. p. 284. 1884. 124 
WHITMAN. 
[Vol. I. 
of the macromeres. The earliest entoderm cells are formed in 
precisely the same manner in Clepsine; but here a few cells are 
left to multiply in the macromeres, and these take the form of 
“ entoplasts,” and are to be regarded as the equivalents of the 
large mesenteric cells of Nephelis. The oesophagus of Nephelis 
is probably lined with cells derived from the first-formed entoderm 
cells, precisely as is the case in Clepsine. That there is no 
fundamental distinction between the “ entoplasts ” of Clepsine and 
the entoderm cells of Nephelis is shown by the fact that we have 
both modes of formatio7i in Clepsine, the one passing gradually 
and insensibly into the other. 
Bergh (No. 9, p. 260) explains the above-named difference 
in the position of the vitelline macromeres as the result of a 
retardation in the development of the mesenteron in Clepsine, 
and apparently regards the Nephelis type of development as the 
more primitive. The view presented above appears to me more 
satisfactory. I am not by any means ready to adopt the idea 
that the mode of development in Clepsine is to be regarded as 
a modified or derived form of that seen in Nephelis. It would 
be quite as rational to take just the opposite ground, and main- 
tain that the egg of Nephelis has been derived from one that 
was heavily loaded with food-yolk. The mode of cleavage, and 
especially the persistence of the three macromeres, appears to 
support such a view.1 
Rhynchelmis (Euaxes).— In spite of the many gaps in our 
knowledge of the development of Nephelis, it has been easy to 
find a close and interesting parallel between it and Clepsine in the 
early phases of the egg. This, to be sure, is no more than 
might have been expected in the case of two forms so closely 
allied, but it is more than could have been conceded without 
first showing how differences of opinion could be reconciled. If 
now we extend the comparison along the same lines to one of 
the Chaitopods, e.g., Rhynchelmis, we shall find the suggestions 
advanced in the foregoing pages corroborated in many points of 
fundamental importance. 
1 The power to elaborate and store nutritive yolk comes and goes with the need; 
but the mode of development induced by the presence of yolk appears to persist, to a 
greater or less extent, even after the yolk has been lost. Such has, in all probability, 
been the case with the mammalian egg, and there is reason to suspect that other 
alecithal eggs have had a similar history. Many anomalies of development may be 
accounted for in this way. No. 1.] 
GERM-LAYERS TAT CLEPSINE. 
125 
The cleavage in Rhynchelmis has been studied with consider- 
able care by Kowalevsky (No. 10, p. 12), and by Vejdovsky 
(No. 14, p. 228). Some of the changes which the egg under- 
goes preparatory to cleavage, as described by Vejdovsky, are 
of great interest, on account of their manifest identity with cer- 
tain remarkable polar phenomena which display themselves with 
great intensity in the egg of Clepsine. I refer to the polar rings 
of hyaline protoplasm, which concentrate at each pole in the 
form of a disc. The first two cleavage-planes divide the egg 
into four macromeres, three of which are nearly equal, and cor- 
respond to the anterior and the two lateral macromeres of Clep- 
sine, while the fourth and largest one represents the posterior 
macromere, and contains most of the hyaline protoplasm of the 
polar discs, precisely as in the egg of Clepsine. Such a close 
correspondence in the primary steps of cleavage, resulting in 
the specialization of one of the four macromeres, affords the 
strongest possible evidence, short of verification by direct obser- 
vation, that the subsequent history of this macromere will be 
essentially the same as that of the posterior macromere in Clep- 
sine. Unfortunately, the observations on this point are too 
incomplete to be decisive; but two important facts may be 
noted which furnish, at least, a partial verification of the view 
here taken. First, the median plane of the embryo bears to the 
first two cleavage-planes relations which are analogous to, if not 
quite identical with, those described in Clepsine; and, secondly, 
a pair of mesoblasts arise from the posterior macromere. 
Remembering that only two teloblasts were hitherto recognized 
in Lumbricus, and that a careful study of the germ-bands from 
surface-preparations has led Wilson to the discovery of neuro- 
blasts and nephroblasts, it is not venturing much to predict a 
similar discovery for Rhynchelmis. In the Hirudinea the meso- 
blasts lie beneath and usually in front of the remaining teloblasts, 
and their relations to the germ-bands are not easily ascertained 
without the aid of sections. Most of the remaining teloblasts 
(neuroblasts and nephroblasts) are very prominent even in the 
living egg. In the Oligochaeta, on the contrary, the mesoblasts lie 
behind the other teloblasts, and are conspicuous in surface-views ; 
(14.) Vejdovsky, Fr. Die Embryonalentwicklung von Rhynchelmis (Euaxes). 
Vorluafige Bemerkungen. 
Silz.-Ber. d. k. bohm. Ges. d. fViss. March, 1886. 126 
WHITMAN\ 
[Vol. I. 
while the neuroblasts and nephroblasts, lying in front of and 
differing but little in size from the cells which they produce, are 
discoverable only by careful microscopical examination. This 
accounts for the fact that they have hitherto been overlooked, 
and for the widespread belief that the mesoblasts are the sole 
proliferators of the germ-bands. The origin of the mesenteron, 
as described by Kowalevsky, is another evidence in favor of 
identifying the posterior macromere in Rhynchelmis with that 
of Clepsine. 
Let us now consider the history of this macromere more in 
detail, in order to see how far observation supports the com- 
parison writh Clepsine. Vejdovsky has extended and corrected 
Kowalevsky’s account of the cleavage in many particulars, and 
I shall therefore be guided by his statements. 
The first cleavage-plane divides the egg into unequal parts, 
the larger of which receives the remnants of the polar discs of 
protoplasm, as in Clepsine. The second cleavage begins first 
on the larger segment, but passes to the small segment before 
the large one is completely divided, so that a three-cell stage 
does not really exist. All this seems at first sight to be in 
perfect accord with the first two cleavages in Clepsine. But 
there is one difference in regard to the second cleavage which 
threatens to upset the whole comparison. This cleavage does 
not divide the still visible remnant of the polar disc, but 
runs to the left of them, while in Clepsine it runs to the right. 
This difference introduces completely new axial relations, 
analogous to, but not identical with, those in Clepsine, as 
will be seen by comparing Diags. 7 and 8. It makes 90° dif- 
ference in the direction of the embryonic axis, for b now becomes 
the posterior macromere, and is destined to play the same rdle 
as the macromere x in Clepsine. The embryonic axis now bisects 
a and b (Diag. 8), instead of c and x, as in Clepsine (Diag. 
7). Thus arises a very serious difficulty in the way of identify- 
ing the macromeres. I see only two ways of meeting this point. 
1. There is, of course, a possibility of error in observation, 
and the liability to error is all the greater, as Vejdovsky does 
not appear to have given any attention to the relations we are 
now considering. 2. If he is right in placing the second cleav- 
age-plane to the left of the disc, it will still be possible to identify 
the macromeres, provided the order of the first two cleavage- No. 1.] 
GERM-LAYERS IN' CLEPSINE. 
Diagrams of the four-cell stage of Clepsine and Rhynchelmis (Euaxes) to show the 
relations of the cleavage-planes to the median plane of the embryo. 
Diag. 7. — Clepsine. The axis of the embryo bisects two opposite macromeres 
(c and x). 
Diag. 8. — Rhynchelmis (constructed after Vejdovsky). The embryonic axis here 
bisects opposite macromeres, but stands at right angles to the position shown in 
Diag. 7. 
a, c, b, x, the four macromeres. 
d, the polar disc of hyaline protoplasm, which marks the posterior macromere. 
I — I, first cleavage-plane; II — II, second cleavage-plane. 
planes in Rhynchelmis is the reverse of that in Clepsine. That is, 
if the first and second cleavage-planes in Rhynchelmis correspond 
respectively, to the second and first in Clepsine, there will be 
a complete correspondence of macromeres and axial relations 
in the two eggs. The appearances are so strongly in favor of 
the identity of the posterior macromeres, that either alter- 
native appears to me more acceptable than the conclusion 
that there is a radical difference in axial relations that bear all 
the outward marks of being absolutely identical. I shall pro- 
ceed on the assumption that the difficulty just considered arises 
from an error of observation, and that the relations between the 
median plane of the embryo and the cleavage-planes are pre- 
cisely the same in both cases. Future observation must deter- 
mine whether this position is well chosen. 
The cleavage of the posterior macromere is described by 
Vejdovsky in the following words: — 
“ In den nachfolgenden Vorgangen spielt die hintere Makro- 
mere die wichtigste Rolle, in der, wie bemerkt, das Protoplasma 128 
WHITMAN. 
[VOL. I. 
von beiden Polen sich concentrirte. Nachdem namlich die 
ersten 4 Mikromeren ihre definitive und gleiche Grosse erlangt 
haben, knospet aus der hinteren Makromere eine grossere, aus 
Protoplasma bestehende Zelle, die, wie die vertikalen Langs- 
schnitte zeigen, aus dem gemeinsamen Protoplasmanest ihren 
Ursprung hat und beziiglich der Grosse zwischen der des Mikro- 
meren und Makromeren steht; somit werden wir sie als Meso- 
mere bezeichnen. 
“ Dieselbe verdrangt die inzwischen vermehrten Mikromeren 
mehr nach vorne und bald darnach bildet sich in derselben 
Weise aus der hinteren Makromere, beziehungsweise aus deren 
Protoplasma die zweite Mesomere, die sich hinter der ersten 
stellt, und schliesslich entsteht die dritte Mesomere, welche in 
Bezug auf die Grosse und Gestalt den vorderen zwei vollstandig 
gleich ist. Mikromeren sind bereits zahlreich zu beiden Seiten 
und nach vorne vorhanden. Die Langsschnitte durch dieses 
Stadium beweisen, dass die hintere Makromere bereits des Proto- 
plasma entbehrt, indent dasselbe zur Bildung der Mesomeren ver- 
wendet wurde. 
“ Die vorderen zwei Mesomeren bleiben eine Zeit lang unver- 
andert, wahrend die dritte, hintere Mesomere sich mehr der 
Quere nach ausbreitet und schliesslich sich in zwei neue, gleicli 
grosse Mesomeren theilt, die bald zu der urspriinglichen Grosse 
ihrer Mutterzelle heranwachsen. Bald darnach theilen sich in 
der Langsaxe auch die zwei vorderen Mesomeren, so dass zwei 
Reihen von je drei gleich grossen Mesomeren entstehen, die aus 
der Umgebung der inzwischen stark sich vermehrenden Mikro- 
meren hervortreten. Die vorderen zwei Mesomeren theilen sich 
nun — immer in der Langsaxe — in zwei, dann in vier Zellen, 
die aber nicht wachsen, sondern durch weitere Theilung die 
Grosse der Mikromeren annehmen. An solchen Eiern treten 
nur die vier hinteren Mesomeren hervor. Dieselben Theilungs- 
vorgange wiederholen sich aber bald auch an dem zweiten 
Paare der jetzt vorderen Mesomeren und somit zerfailen diesel- 
ben in eine Anzahl der Mikromeren, wahrend das hinterste Paar 
der Mesomeren sow ohl ietzt a Is auch spater bei der spateren 
Furchung der Makromeren unverandert bleiben und als zwei 
weisse, stark gewolbte Kugeln dem hinteren Ende der Mikrome- 
ren aufsitzen.” (No. 14, pp. 234-335). 
It is evident from this description that Vejdovsky’s “ meso- No. 1.] 
GERM-LAYERS IN CLEPSINE. 
129 
meres” correspond to the teloblasts of Clepsine. The two 
posterior mesomeres represent the mesoblasts, while the four 
anterior mesomeres probably represent the neuroblasts and the 
nephroblasts. Vejdovsky lets the anterior mesomeres divide 
up into micromeres, and fails to connect them with the nerve- 
cord and the nephridia. The discovery of a full set of telo- 
blasts in Lumbricus makes it almost certain that such cells are 
concerned in the formation of the germ-bands of Rhynchelmis, 
and the mesomeres appear to be the only cells that could here 
be so identified. Allowing that the mesomeres admit of this 
interpretation, it is clear that the posterior macromere in Rhyn- 
chelmis fulfils the same ends as does the posterior macromere 
in Clepsine. The mesomeres differ from the teloblasts in num- 
ber and in mode of origin, but agree with them in position, 
derivation, and purpose. 
According to Vejdovsky (p. 235), the remnant of the pos- 
terior macromere, after the production of the three primary 
mesomeres, takes part with the other three macromeres in 
the formation of the mesenteron. I have not traced any 
entodermic elements to this macromere in Clepsine; but, in 
view of the fact that the mesoblasts persist for some time after 
they cease to contribute to the germ-bands, it would not be 
strange to find that their final products were entoplasts. Should 
this turn out to be the case, the origin of all the germ-layers 
would be as nearly the same in both eggs as one could well 
expect. 
The origin of the mesenteron in Rhynchelmis has been well 
described and figured by Kowalevsky. A glance at his figures 
is sufficient to show that the mode of origin is essentially the 
same as in Clepsine. There are three primary entoblasts (four 
with the remnant of the posterior macromere), differing from 
those in Clepsine only in breaking up into a number of second- 
ary entoblasts. The formation of entoplasts takes place in the 
same manner as in Clepsine, and the residual yolk is finally in- 
closed in the mesenteron. The whole process is so similar to 
what I have described in Clepsine that I have been sur- 
prised to find Hoffmann and Bergh, who must be familiar with 
the facts, disposed to reject my conclusions. It is hardly 
necessary to add that this mode of origin is very common 
among Arthropods; and that here, as in Rhynchelmis, the 130 
WHITMAN. 
[Vol. I 
similarity extends to the oesophageal portion of the alimentary 
canal. 
It is an interesting fact that in those cases where the major 
portion of the mesenteron passes through well-marked stages of 
differentiation, beginning with one or more primary vitelline 
spheres, then, with or without further subdivision of these 
spheres, taking the form of free peripheral nuclei imbedded in 
protoplasm with no defined cell boundary-lines (entoplasts), 
and finally assuming the form of a distinct lining epithelium; 
there is always an anterior portion, which arises quite early, and 
which, from the first, consists of distinct cells, often scarcely 
distinguishable from the cells of the ectoderm and mesoderm. 
Numerous cases might be cited, but I must here limit the com- 
parison to Rhynchelmis. On this point I may refer again to 
Vejdovsky. Speaking of the origin of the entoderm at the an- 
terior end of the embryo, he makes the following remarks: — 
“ Es bildet sich hier eine Gruppe dicht neben und anein- 
ander liegenden Hypoblastzellen, die der Dotterkiigelchen vollig 
entbehren und sich ebenso intensiv roth wie die Keimstreifzellen 
tingiren. Den ersten Anfang derartiger Hypoblastzellen kann 
man bereits in dem Stadium finden, als die Mesomeren sich 
einzustulpen1 und die ersten Keimstreifzellen zu produciren 
beginnen. Aus den derart differencirten Hypoblastzellen bildet 
sich spater die Epithelschicht des Anfangstheiles vom Mittel- 
darme — der Oesophagus. (No. 14, p. 237). 
Branchiobdella. — In a recent paper on the embryology of 
Branchiobdella, Salensky (No. 15, p. 1) has compared this form 
with Clepsine and Nephelis. The paper is a most welcome ad- 
dition to our knowledge of this interesting parasite; but the 
points of chief interest here — the cleavage, axial relations, and 
origin of the germ-layers — are too imperfectly worked out to 
admit of a close comparison with Clepsine. 
Salensky finds on one pole of the egg a clear spot, which 
1 The “ Einstulpung ” of the mesomeres, described by Vejdovsky, is comparable 
with the movements which result in imbedding the teloblasts of Clepsine in the vitel- 
line spheres. I believe this imbedding process is best understood as a part of the 
invaginatory movement described in my former paper (No. I, pp. 58-59). The 
mesoblasts lie at the hind end of the blastopore, between the entoblast and the ecto- 
blast, and their products are carried inward between the two primary layers. 
(15.) Salensky, W. Developpement de Branchiobdella. Arch, de Biol. vi. 
Fuse. I, p. 1. 1885. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
131 
corresponds in position to the archiamphiaster (“ amphiaster de 
rebut ”), and which is traversed by the first cleavage-groove. 
This pole is correctly identified with what I have called the 
“ oral pole ” in Clepsine. In view of these facts it is very re- 
markable that Salensky should regard the first cleavage-groove 
as equatorial. I fail to see a single fact which could be urged 
in support of such a view. The relation of this groove to the 
“ clear spot” is conclusive evidence that it is meridian, precisely 
as it is in the Hirudinea and other annelids. If further proof be 
required, it may be found in the development of the eight-cell 
stage, which corresponds in every prominent feature with the 
same stage in Clepsine, Nephelis, and Rhynchlemis. This is a 
point of primary importance; for if the first cleavage were equa- 
torial, the subsequent cleavages would be just as little comparable 
with those of Clepsine as the first, and the axial orientation 
would be radically different in the two cases. Precisely how 
the embryonic axis is related to the first two cleavage-planes is 
not clear; but, judging from Salensky’s figures, it appears to 
be the same as that of the types before considered. There is a 
large macromere, from which the first micromere arises, and 
which divides, asymmetrically, prior to the division of the other 
three macromeres. It is this macromere that I would identify 
with the “ posterior macromere ” in Clepsine. According to 
Salensky (pp. 20-21) the two segments into which this ma- 
cromere divides, participate equally in the formation of all the 
germ-layers. That is, they are not destined to play unlike parts, 
as they do in Clepsine, the one, representing a primary meso- 
blast, the other a neuro-nephroblast, but each alike gives rise 
to entodermic, mesodermic, and probably ectodermic elements. 
“ Elies donnent naissance aux cellules niesoentodermiques et ne 
represented point des ebauches speciales ni du mesoderme, ni 
du systeme nerveux, comme c’est le cas chez Clepsine.” 
There appears to be the same confusion here as we found in 
Nephelis, in regard to the origin of the entoderm and meso- 
derm. The earlier “ meso-entodermic ” cells in Branchiobdella 
appear to correspond to the first deep cells which arise beneath 
the micromeres in Clepsine and Nephelis, and which, as I have 
shown, are purely entodermic. Salensky has found cells 
which are unmistakably the homologues of the teloblasts of 
Clepsine; but he has failed to trace their origin and to deter- 132 
WHITMAN. 
[Vol. I. 
mine the special part which each plays in the formation of the 
embryo. Although he has thus left it impossible to distinguish 
the different kinds of teloblasts, we may safely assume that they 
represent mesoblasts, neuroblasts, and nephroblasts, and that 
they all originate from the large posterior macromere. Allow- 
ing that the teloblasts are here similar in origin and character to 
the teloblasts of Clepsine, Nephelis, and Lumbricus, it follows 
that the entoderm must arise, chiefly at least, from the other 
three macromeres. The chief difference, then, between Branchi- 
obdella and Clepsine, in respect to the entoderm, would lie not 
in the source, but in the mode of origin. In the former the 
cleavage process is continued to the end, while in the latter it 
ceases with the formation of the primary macromeres, the work 
then being completed by the intermediation of entoplasts. 
In regard to the extent to which the epithelium of the ali- 
mentary canal is of entodermic origin, Salensky remarks (p. 
56) : “ Ce canal tout entier, a l’exception des parties insigni- 
fiantes avoisinant la bouche et l’anus, est exclusivement forme 
par l’entoderme.” Speaking of the oesophagus, he says (p. 58) : 
“ Le processus de revolution de l’oesophage, chez Branchiob- 
della, demontre clairement que tout l’epithelium de cette partie 
nait exclusivement aux depens de l’entoderme.” The same 
will be shown to hold true in Clepsine. 
Salensky (No. 15, p. 19) has misunderstood my statements 
with reference to the relation of the embryonic axis to the 
main axis of the egg. The cephalic lobe is very nearly cen- 
tred on the upper pole of the egg in Clepsine, and the mouth 
arises at, or at least very near, this pole. It does not follow, 
however, that because the mouth is located at the oral pole, 
the posterior end of the embryo must be found at the opposite, 
or aboral pole. A glance at my figures will show that the 
two ends of the embryo are at first very near together, the cau- 
dal end itself lying just behind the area occupied by the four 
primary micromeres. The axis of the embryo may, therefore, 
be said to be at right angles to that of the egg, as it is in 
Branchiobdella and Nephelis.1 
11 cannot agree with Salensky that the embryology of Branchiobdella establishes 
its title to be ranked among the Hirudinea. Both its development and adult structure 
appear to me to sustain the opinion, now held by most authorities, that it stands 
nearer the Oligochaeta than the Hirudinea. No. 1 ] 
GERM-LAYERS IN CLEPSINE. 
133 
The results of my study on Clepsine parasita are supple- 
mentary to those obtained on C. marginata (No. 1, pp. 57-58, 
66-72) and C. complanata. For the sake of clearness as well 
as completeness, I have decided to give both the earlier and 
the later observations a place in the present paper. The exist- 
ence of free nuclei in the surface of the three entoblasts, a, b, c, 
first pointed out in my paper on Clepsine, has been confirmed 
by Bergh and Nusbaum. The early history of these nuclei is 
given in the following citation: — 
Observations. 
“ About the time the germ-bands begin to form, a number of 
free nuclei appear in the surface of the entodermal blastomeres, 
a, b, c. These nuclei are very distinct in the egg of C. com- 
planata, and it is remarkable that they have so long escaped 
observation. They appear like dark spots in the opaque yolk, 
just as the nuclei of the neuroblasts or of the blastodisc. They 
are oval, oblong, or biscuit-shaped, and measure .02 to .05 mm. 
At the time of appearance they number three to four in each 
blastomere, two or three of which occupy the position seen in 
the figure (No. 1, Fig. 37), while the others are near the lower 
pole. They are encircled by white rings, such as are generally 
seen around the nuclei of the neuroblasts. The substance of 
these rings is the same as that of the white borders of the rings 
and ring-discs. 
“ I have seen these nuclei pass through the successive forms 
of a dividing amphiaster. They multiply rapidly, and, in the 
stage of Fig. 38, are scattered over the whole outer surface of 
the blastomeres. In the following stages they can also be seen 
on the upper faces of a, c, and b, through the thin ectoder- 
mal layer. By the time the germ-bands are fully united they 
are very numerous, and much smaller than at first. 
“Whence come these nuclei? In the stage of Fig. 35 they 
are not to be seen. A horizontal section of this stage (Fig. 
80) shows that each blastomere possesses a single nucleus. 
The nucleoplasm has a somewhat stellate form. The rays vary 
in length, sometimes reaching to the irregular circular outline 
of the nucleus. The same condition has been described in 
Nephelis by Biitschli. Fig. 61 represents one of these nuclei 
in a little earlier phase. The nuclei now lie nearer the inner 134 
WHITMAN1 
[Vol. I 
than the outer faces. Fig. 83 represents a horizontal section 
of the stage of Fig. 37, which passes beneath the neuroblasts 
and the blastodisc. Here only two nuclei were hit, but these 
lie near the outer faces of the blastomeres. The nuclei of the 
blastomeres then pass from their original central position to the 
periphery, and can here be seen in the living egg.” (No. 1, 
PP- 57-58.) 
I was unable to trace these nuclei directly through all their 
later stages of multiplication, but various facts led me to con- 
clude that they gave rise to the mesenteron. Between the last 
stage in which I could recognize these nuclei and that in which 
the mesenteron became distinct, a number of stages intervened 
in which I was unable to demonstrate their existence. This 
failure, due to imperfections in methods of preparation, was a 
source of doubt, in spite of the many indications which made 
it appear almost certain that they represented the mesenteron. 
I satisfied myself that the entoderm could not have its origin 
in elements derived from the germ-bands, for all these elements 
were plainly turned to other uses. Besides, the earliest ap- 
pearance of the entoderm cells,—their loose and irregular 
order in the periphery of the yolk, — pointed to their origin 
from the free superficial nuclei of earlier stages. After describ- 
ing the earliest appearance of the mesenteron (p. 66), I stated 
my conclusion in the following words: — 
“ These superficial nuclei go on multiplying by division dur- 
ing the whole period of the epiboly. Finally they are seen as 
mere white dots scattered over the entire surface of the yolk. 
Six to seven days after exclusion the entoderm cells make their 
appearance as clear cells with small nuclei, in the periphery of 
the yolk already cut up into compartments by the septa. 
What hypothesis is more probable than that these cells origi- 
nate from the free nuclei? My sections have convinced me 
that these entoderm cells arise in the surface of the yolk, and that 
they do not originate in the products of the blastodisc ” (p. 67). 
The condition of the mesenteron represented in Figs. 93 and 
95 of my former paper, is very similar to that shown in Nus- 
baum’s Figs. 13 and 14, PI. II. 
With regard to the origin of the free nuclei from the primary 
nuclei of the entoblasts, I have nothing to add to the state- 
ments above cited. The observations which follow begin with No. 1.] 
GERM-LAYERS IN CLEPSINE. 
135 
an early stage of the embryo, before the germ-bands have 
reached the equator of the egg, and the illustrations for the 
earlier phases described (Figs. 1-5) are drawn from eggs of C. 
complanata, obtained at Naples. 
Figure 1 represents a surface view of the egg, with the germ- 
bands in an equatorial position, before they have united at the 
cephalic end. The white patches (enp) seen in the yolk, be- 
low the germ-bands, are nucleated masses of finely granular 
protoplasm, to which I have given the name entoplasts. The 
existence of such bodies was entirely overlooked by Hoffmann 
in his first paper; but in his second paper he describes them 
as “Protoplasmajlecke,” and devotes considerable space to re- 
futing my interpretation of them. Nusbaum (No. 8, p. 2) 
carefully translates Hoffmann’s description, as if it were some- 
thing original, using “ Hots protoplasmiques ” as the equivalent 
of Hoffmann’s term. 
Sections of the egg (Figs. 2—5) show that many of the ento- 
plasts have not yet reached a peripheral position. In a 
sagittal section of the head (Fig. 4) we see scattered entoplasts 
{enp) in the yolk, and external to them some large entoderm 
cells (en). Some of these cells are only faintly or imperfectly 
circumscribed by boundary lines, representing transitional 
phases between the entoplast and the clearly defined entoderm 
cell. The differentiation of the entoplasts is going on more 
rapidly in this region than elsewhere, and it is here that they 
first assume an epithelial character. Another sagittal section 
of the same egg, nearer the median plane, is seen in Fig. 3. 
Here the same large entoderm cells are seen beneath the head 
{cl), and a few neighboring entoplasts, not yet delimited, against 
the yolk. This one section shows fourteen entoplasts, only 
three of which could have been seen from the surface. The 
rest lie beneath the cephalic lobe (cl) and around the meso- 
blast x, which is here nearer the anterior than the posterior 
end of the egg. In the transverse section (Fig. 2) fewer ento- 
plasts are seen, — four in a, three in c, and one in b. The pro- 
toplasm surrounding the two nuclei at the upper angle of c is 
continuous with the ectoblast, which raises the question whether 
contributions to the latter have continued up to so late a stage. 
I am not able to decide the question, but I am inclined to think 
that the macromeres cease to proliferate ectoblastic elements 136 
WHITMAN. 
[Vol. I. 
when the formation of free nuclei begins. Near the upper 
angle of xy, between xy and a, is seen a well-defined entoderm 
cell. It is rare to find the entoplasts assuming the cell form 
at such a depth. Fig. 5 (a horizontal section in the plane 
of the arrow 5-5, Fig. 3) shows entoplasts at different depths, 
one of which has become defined as a cell {en), while another 
near by shows faint indications of its future outline. 
Passing now to the stage in which the germ-bands have 
united for about one-half their length, we find all the entoplasts 
in the periphery of the yolk, and a marked advancement 
already made in the development of the cephalic end. Beneath 
the stomodaeal thickening (Fig. 20, PI. VI.) is seen a mass of 
large clear cells {en), as yet presenting no definite form and 
giving no indication of their future histological character. 
They are easily identified with the entoderm cells beneath the 
cephalic lobe in Figs. 3 and 4. 
In a little later stage (Fig. 21, en), when the germ-bands are 
nearly closed, these cells are smaller, and appear to be taking 
a more definite shape, as they become more sharply delimited 
from mesodermic (m) and neural {sup, oe, g) elements. We 
now distinguish an anterior axial portion, forming a solid pad 
beneath the stomodaeum, and a posterior portion {sgl), con- 
sisting of larger cells, stretching towards the dorsal and ventral 
sides. Those of the dorsal side are more deeply stained than 
those of the ventral side. In several instances I have seen a 
column of these clear cells extending through the centre of the 
stomodaeum, as shown in Fig. 21 ; and this leads me to 
believe that the canal of the proboscis is lined throughout 
with cells of entodermal origin. The condition shown in Fig. 
28 fully bears out such a view. Along the ventral side of the 
yolk (Fig. 22) there is a peripheral layer of coarsely granular 
protoplasm (“ conche protoplasmique granuleuse ” of Nus- 
baum), feebly stained, in which may be seen free nuclei {enp). 
Similar nuclei are found on the dorsal side, but there they are 
less numerous, and the peripheral layer of granular, uncolored 
protoplasm is not present. 
By the time the germ-bands are completely closed, and the 
embryo is ready to leave the egg membrane (Fig. 28), the 
entoderm of the oesophageal region has differentiated into small, 
axially placed cells, of a distinctly epithelial character, and No. 1.] 
GERM-LAYERS /AT CLEPSLVE. 
137 
larger cells, destined to form the several pairs of massive cell- 
groups, known as the salivary glands (sgl), which are later 
found on the dorsal and ventral sides of the pharynx, with canals 
leading into the extreme hind end of the proboscis. The 
epithelial portion extends through the proboscis, now distinctly 
marked off from the rest of the stomodaeum, and reaches back- 
ward between the dorsal and ventral masses of glandular cells. 
There is still no lumen recognizable in any portion of the 
oesophagus, but the entodermal epithelium is so well differenti- 
ated in color and general appearance, and agrees so perfectly 
with the conditions seen later in the posterior portions of the 
alimentary canal, that there is not the least difficulty in distin- 
guishing it from the other embryonic tissues associated with it. 
On the dorsal side of the yolk the entoderm is still represented 
by entoplasts (enp), but on the ventral side, by an extremely 
thin layer of epithelial cells (en), which would be easily over- 
looked, except for the strongly stained oval nuclei. In my study 
of C. marginata I failed to recognize this very obscure layer, 
and thus overlooked a stage of development which connects the 
entoplasts with the epithelium derived from them. The same 
condition is seen along the middle region of the ventral side, as 
shown in Fig. 26, en, but the layer vanishes a little farther on, 
behind which point we find scattered entoplasts (Fig. 27, enp). 
The gradual transition from this flattened epithelium into 
columnar epithelium is well shown in Fig. 25, which represents 
the dorsal half of one of the anterior caeca in C. marginata, 
nine days after hatching. 
Recapitulation. — The history of the mesenteron may now 
be recapitulated. 
I. The earlier entoderm cells arise beneath the cephalic lobe, 
and are probably budded off from the entoblasts, a, b, c, as 
distinct cells, precisely as inNephelis. But to these earlier and 
regularly formed cells are soon added others, which appear 
first as entoplasts, so that it is impossible to draw any line of 
distinction based on the mode of origin. 
2. The larger portion of the mesenteron, embracing the 
whole alimentary tract, with the exception of a small, anterior 
(oesophageal) portion, passes through the following stages of 
development. The first stage is represented by the three large 
macromeres, or entoblasts (a, b, c) ; the second by entoplasts 138 
WHITMAN'. 
[Vol. I. 
(each represented by a nucleated mass of protoplasm without 
cell-boundary) ; the third by an exceedingly thin layer of flat- 
tened epithelium ; and the fourth by a columnar epithelium. 
3. The development of the mesenteron begins at the anterior 
end, and progresses towards the posterior end, but more rapidly 
along the ventral than the dorsal side. 
4. The phases of development are essentially the same as in 
Rhynchelmis, the chief difference being that, in the latter, the 
three primary entoblasts a, b, c, split up into secondary ento- 
blasts before the entoplastic phase appears. 
5. The history of the mesenteron in Nephelis is very imper- 
fectly known, but there is nothing in the observations thus far 
published which appears to be irreconcilable with the results 
obtained in Clepsine. The development in Clepsine is more 
complicated, owing to the larger amount of food-yolk. It is 
doubtful whether the entoplastic phase is represented in 
Nephelis. 
6. It is possible that the residual mesoblasts (the remnants 
left after the completion of the germ-bands) contribute to the 
formation of the mesenteron. Such a termination of their his- 
tory has not been ascertained, but is suggested by the fate of the 
posterior macromere in Rhynchelmis. 
7. The proboscis — the homologue of the muscular pharynx 
of the Gnathobdellidae — is lined with cells of entodermal 
origin. The rest of the proboscis, together with the proboscid- 
ial or pharyngeal pocket, is derived from the stomodaeal 
thickening of the ectoderm. 
8. A knowledge of the history of the teloblasts clears up 
many obscurities in regard to the origin and relations of the 
germ-layers, particularly the entoderm and mesoderm. If the 
precise origin of the teloblasts be known, that of the entoderm 
may be inferred, and vice versa. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
139 
III. THE ECTODERM AND ITS PRODUCTS. 
1. The Ectoderm. 
The origin of the first four ectodermic cells (micromeres) has 
been described under the head of cleavage and axial relations. 
By the addition of numerous other micromeres, arising, mainly, 
from the anterior and the lateral macromeres, a sort of blasto- 
disc is gradually formed, centered at the upper pole of the egg. 
This blastodisc is not wholly ectodermic, for a few of its deeper 
cells, as we have seen, represent the earlier entoderm cells, as 
was first suggested by Bergh. The superficial, ectodermic por- 
tion of the blastodisc gives rise to the epidermal layer and its 
derivatives, the stomodaeum, sense-organs, etc. 
The ectoderm includes, in addition to the superficial portion 
of the blastodisc, all the teloblasts, except the two larger and 
and deeper ones, which represent mesoblasts. The grounds for 
regarding the eight smaller teloblasts as part of the ectoderm 
are the following: 1. They have at the outset a superficial 
position at the hind edge of the blastodisc. 2. Two of them 
give rise to the ventral nerve-cord. 3. In Lumbricus {vide 
Wilson) they lie in, and plainly form a part of, the general ecto- 
derm. 
2. The Ventral Nerve-chain. 
In a preliminary paper (No. 16) I have already stated that 
the nerve-chain of Clepsine first appears in the form of two sim- 
ple, unsegmented rows of cells; and, further, that each row 
is the product of a single cell, the neuroblast. At the time 
this fact was announced nothing of the kind was known in any 
other animal; and Nusbaum, the latest authority on Clepsine, 
had just arrived at an entirely different conclusion, and one 
altogether more in harmony with traditional views. A similar 
discovery has since been made by Wilson in Lumbricus, and, 
fortunately, the evidences in both cases can now be presented 
side by side. The subject is one which has received a good 
deal of attention, and given rise to considerable discussion. 
Before reviewing the opinions of other writers, or giving my 
(16.) Whitman, C. O. The Germ-Layers of Clepsine. Zool. Anz. No. 
218. 1886. 140 
WHITMAN. 
[Vol. I. 
own observations, it will be well to consider briefly some points 
respecting the germ-bands. 
Use of the Term Germ-bands. — It is important to settle 
at the outset precisely what is to be understood by the term 
“ germ-bands.” Keimstreifen, the German equivalent, is usually 
restricted to the strata derived from the teloblasts, the epider- 
mal layer being excluded. It is, furthermore, generally believed 
that the germ-bands of the annelids embrace only mesoblastic 
elements. Although appearances may often favor such a re- 
stricted use of the term, we cannot so limit it in all cases. The 
idea that the germ-bands are purely mesoblastic has already 
led to much confusion. In the Hirudinea it is perfectly certain 
that ectoblastic elements must be included, and hence the mat- 
ter is not in the least simplified by excluding the epidermal 
stratum. In Lumbricus, where, according to Wilson, the neuro- 
blasts and nephroblasts are at first ordinary ectoblastic cells, 
and where, after sinking beneath the surface, they remain im- 
bedded in the epidermal layer, it is obvious that this layer can- 
not well be held to be distinct from the elements of the germ- 
bands. According to Bergh (No. 9) the “ definitive epidermis ” 
in the Gnathobdellidae arises from what I shall call the neuro- 
nephric stratum of the germ-bands. I cannot, therefore, fol- 
low Kowalevsky, Biitschli, Hatschek, Balfour, Goette, and 
numerous other writers in the use of “ mesoblastic bands ” as 
the equivalent of germ-bands, nor can I accept the alternative 
offered by Bergh (No. 9, p. 285), which denies the homology 
of the germ-bands. The moment we undertake to exclude 
ectodermic elements, the basis for homology is sacrificed, and 
the door is open to endless confusion. I shall, therefore, include 
in the term germ-bands both the “ mesoblastic bands ” and the 
superjacent ectodermic strata. It must be remembered, also, 
that the term, as here used, has reference only to the body of 
the embryo. 
Germ-bands of the Head. — The relations of the cephalic 
lobe to the germ-bands have not yet been made clear in Clep- 
sine. In Aulostoma, Bergh (No. 9) finds in the head two dis- 
tinct germ-bands, which arise independently of each other and 
of the germ-bands of the body. The head-bands (“ Kopf- 
keime”) contain epidermal, neural, and muscular elements, and 
are regarded as homodynamous with the trunk-bands (“ Rumpf- No. 1.] 
GERM-LAYERS IN CLEPSINE. 
141 
keime”). Leuckart (No. 17, p. 706) described the head- 
bands in Hirudo as “ Zwei einfache seitliche Anschwellungen, 
die rechts und links vor der Munddffnung gelegen sind und 
durch eine ziemlich lange Commissur sowohl unter sich, als 
auch mit den jetzt hornfdrmig ausgezogenen Vorderenden der 
Unterschlundganglienmasse zusammenhangen.” Semper (No. 
18, p. 215) was the first to point out two distinct head-bands 
(“ Sinnesplatten,” “ Kopfkeimstreifen ”) in the Hirudinea. “ Bei 
Clepsine, wie bei Nephelis, der Schlundring und das dorsale 
Schlundganglion entsteht gerade so wie bei Hirudo, durch Ver- 
wachsen zweier Sinnesplatten.” He insists, however, that 
these sense-plates (p. 247) are “ echte Kopfkeimstreifen, von 
deren Bildungszellmasse nur ein Theil zum Nervensystem wird, 
wahrend ein anderer Theil sich in die iibrigen Organe des 
Kopfes, vor Allem in die mit dem Schlunde sich verbindenden 
Organe umwandelt.” 
As above remarked, I am not prepared to discuss the question 
as to the existence of two independent germ-bands in the 
cephalic lobe. I have stated (No. 1) my conviction that this 
lobe is formed at the expense of the first four micromeres. 
Should there prove to be two head-bands, as maintained by 
Semper and Bergh, it would be an interesting problem to deter- 
mine whether they hold the same relation to the first four 
micromeres as the germ-bands of the trunk to the teloblasts. 
If such a relation could be demonstrated, the homodynamy of 
head and trunk would be placed in a very instructive light. The 
entire embryo, with exception of the mesenteron and epidermis, 
would then be built up in fundamentally the same manner, at 
the expense of terminal blastomeres, ten teloblasts, and four 
micromeres, or acroblasts. The theoretical difficulty presented 
by independent rudiments for the head and trunk could then be 
disposed of. The temporary separation of the rudiments might 
be regarded as an accident of their present mode of origin 
rather than as an expression of their primitive relations. Their 
real and essential unity is discoverable in the history of the 
originating blastomeres. It will be remembered that the pos- 
terior macromere produces not only the mesoblasts, neuroblasts, 
(17.) Leuckart, R. Die Menschlichen Parasiten. I. 1863. 
(18.) Semper, C. Die Verwandtschaftsbeziehungen der gegliederten Thiere, 
Arb. a. d. Zool.-Zoot. Inst, in Wurzburg. III. 1876. 142 
WHITMAN. 
[Vol. I. 
and nephroblasts, but also one of the four primary micromeres. 
The teloblasts stand thus in the direct line of descent with the 
acroblasts, and are at first in close contact with them. The full 
significance of the teloblasts and their original relations can only 
be made clear by comparison with the larval forms of other 
annelids. Farther on I shall indicate briefly some points in this 
comparison. 
Germ-bands of the Trunk.—Each germ-band consists of 
three distinct layers: (1) A thin epidermal layer, (2) a 
neuro-nephric layer, and (3) a mesoblastic layer. The character 
and relative positions of these layers may be seen in Figs. 2, 6, 
and 7, PI. IV. The epidermal layer (ep) consists of flattened 
cells, more deeply stained with osmic acid than the underlying 
strata. The neuro-nephric layer is represented by four longi- 
tudinal rows of cubical or oval cells (nc, nph, and m'), as is best 
seen in surface views (Fig. 8, PI. V.). The mesoblastic layer 
(m) consists of large, rounded, or polygonal cells, two or more 
deep, filling the space between the neuro-nephric layer and 
the yolk. 
Origin of the Ventral Nerve-Chain. — As my observations 
on the origin of the nerve-chain contradict those of Kowalevsky 
and Nusbaum, and as they do not confirm the anticipations of 
such clear-sighted embryologists as Balfour, I can hardly do 
justice to the subject without dealing briefly with its historical 
side. 
(a) Historical and Critical. — Filippi (No. 19, p. 23), the 
earliest writer on the embryology of Clepsine, tells us that it 
is impossible to trace the origin of individual organs, owing to 
the small size of the embryos. 
Grube (No. 2, p. 35) derived the nerve-cord from the germ- 
bands (“ Bauchwiilsten ”), the defective technique of the times 
not enabling him to reach more definite results. 
In the posthumous work of Rathke, edited and revised by 
Leuckart, the nerve-chain is said to be formed from the median 
part of the germ-bands (“ Bauchplatten ”). This result, obtained 
long before the introduction of the microtome, comes much 
nearer the truth than the statements of Kowalevsky or Nus- 
baum. Without the aid of sections, the inner stratum of the 
(19.) Filippi, F. de. Sopra l’Anatomia e lo Sviluppo delle Clepsine. Pavia, 
1839. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
143 
germ-bands could not be distinguished from the neuro-nephric 
stratum, and this is the principal failure in Rathke and Leuck- 
art’s description. They recognized four rows of cells, each 
terminated by a teloblast, but were mistaken in supposing that 
these rows alone made up the entire thickness of the bands. 
Their conclusion is stated in the following words (No. 3, p. 94) : 
“ Wie bei Nephelis, so scheidet sich auch bei den Clepsinen ein 
jedes dieser Tafelchen [metamere] in zwei neben einander 
liegende Halften, von denen die eine, die der Medianlinie des 
Korpers anliegt, zu einem Theile des Bauchmarkes wird, wahrend 
sich die andere in ein plattes und diinnes Biindel quer verlauf- 
ender Muskelfasern entwickelt.” 
In Leuckart’s celebrated work on Human Parasites (No. 17, 
pp. 702-703), the longitudinal commissures and the lateral 
ganglia of the nerve-chain are described (in Hirudo) as arising 
independently of each other, the ganglia being formed from 
the median portions of the bands, while the commissures are 
derived from a “ helle Furche” (presumably ectodermic) which 
separates the bands. “ Der Primitivstreifen besteht aus zwei 
Halften, die trotz ihrer dichten Anlagerung in der Mittellinie 
durch einen schmalen Zwischenraum getrennt sind. Bei durch- 
fallendem Lichte erkennt man hier eine helle Ftirche, die ziem- 
lich bald, von den sich rasch entwickelnden Langsfasern, ein 
etwas streifiges Aussehen annimmt.” 
The origin of the ganglia from the median parts of the germ- 
bands, subsequent to the division into metameres, and of the 
commissures from the median groove, is then described as fol- 
lows : “ Die Entwicklung der Ganglien geht von dem Innenrande 
der einzelnen Felder [metameres] aus und geschieht dadurch, 
dass dieser zappenformig in die Langsfurche zwischen den beiden 
Halften des Primitivstreifens hineinwachst, sick an den hier, wie 
erwahnt, schon friiher vorhandenen Langsfaserstrang anlegt und 
schliesslich von der iibrigen Zellenmasse des Feldes abtrenntd 
Metschnikofif (No. 7, pp. 671-673) was the first to recognize 
three distinct strata in the germ-bands, and also the first to de- 
termine the origin of the nerve-chain from the neuro-nephric 
stratum. He was wrong, however, in supposing that this whole 
stratum is converted into the nerve-cord. His brief description 
runs thus: “ Bei dem ersten Erscheinen der beiden Keimstreifen 
bestanden dieselben bereits aus drei Keimblattern. Das oberste 144 
WHITMAN. 
[Vol. I. 
Blatt erschien in Form eines diinnen Hautchens, welches den 
ganzen Embryo von alien Seiten umgab. Die beiden anderen 
Keimblatter beschrankten sich bios auf die Keimstreifen. Das 
eine von diesen Blattern, dasjenige namlich, welches unmittel- 
bar unter dem obersten Hautchen lag, bestand aus einer Reihe 
grosser Zellen, welche in vier Reihen in jedem Keimstreifen 
geordnet waren. Das untere, dicht dem Dotter anliegende Blatt 
erschien in Form eines dicken, aus kleinen Zellen bestehenden 
Wulstes. Bei weiterer Entwicklung, zur Zeitwann sich die bei- 
den Keimstreifen in ein Ganzes verschmolzen haben, erfahren 
die Keimblatter wichtige Umanderungen wobei iibrigens das 
oberste diinne Blatt nur eine untergeordnete Rolle spielt. Dieses 
behalt seine urspriinglichen Eigenschaften und erweist sich bald 
als die Epidermis des Embryo. Das zweite Blatt, welches nun- 
mehr aus kleineren Zellen zusammengesetzt wird, bildet dann das 
centrale Nervensystem.” 
Speaking of the germ-layers of Clepsine from a comparative 
stand-point, Metschnikoff remarks, — “ Der Hauptunterschied 
bei Clepsine besteht darin, dass sich das epidermoidale Blatt sehr 
friih von der Nervenanlage absondert. . . . Die beiden ersten 
Keimblatter von Clepsineembryonen werden somit dem oberen 
Blatte des Skorpions und anderer Articulaten entsprechen.” 
Independently of Metschnikoff’s paper, which had escaped my 
attention, I came to precisely the same conclusion (No. i) ; and 
a little later Hoffmann (No. 6, p. 42) repeated my observations 
on this point, without adding to or correcting them. Other 
writers, with a single exception, have failed to get as near the 
truth as this, and their observations have tended to confusion 
rather than enlightenment. 
Bergh is the only one among the more recent writers who 
has made any advancement on the observations of Metschnikoff, 
Rathke, and Leuckart. In the course of his extensive papers 
on the metamorphosis of the Gnathobdellidae, and in several 
reviews, he has stated very briefly the results of studies yet to 
be published, as I am led to infer, on the origin of the nerve- 
chain. According to Bergh, the original epidermal layer is 
lost, and the “ definitive epidermis ” is then formed from the 
lateral portions of what I have called the neuro-nephric stratum, 
while the nerve-cord is formed from the median portion of the 
same stratum. Although “ median portion ” is still an indefinite No. 1.] 
GERM-LAYERS IN CLEPSINE. 
145 
quantity, it is evident that Bergh has made a closer approxima- 
tion to precision than any of his predecessors. Grube is the 
least definite of all; Robin (No. 5, pp. 199, 344) regarded the 
entire germ-bands as the central nervous system. Rathke and 
Leuckart traced its origin to the median portion of the bands; 
Metschnikofif limited it to a single stratum, and Bergh to the 
median portion of the same stratum. 
From the following comments by Bergh it will be seen that 
he thought it impossible to trace the nerve-cord to special 
neuroblasts in Aulostoma, and a little more than impossible ( !) 
in the case of Clepsine : — 
“ Whitman hat die zehn Zellen am Hinterende der Rumpf- 
keime gefunden, welche bei Clepsine durch ganz besondere 
Grosse ausgezeichnet sind; die acht derselben nennt er Neuro- 
blasten, die zwei dagegen Mesoblasten, indem er annimmt, dass 
aus den ersteren nur Nervensystem, aus den anderen nur Meso- 
derm entstehe. Vergeblich sucht man in der genannten Arbeit 
ebenso wie in der Natur selbst irgend einen Beweis fUr diese 
Behauptung, die eben nur eine solche ist. Es ist bei Aulostoma 
'vollkommen unmoglich, die Descendenten jeder einzelnen der 
erwahnten grosseren Zellen fur sich zu verfolgen, und bei 
Clepsine wird die Sache noch viel schwieriger, indem die Rumpf- 
keime hier stark gekriimmt sind. Richtig ist es aber, wenn 
Whitman ganz im Allgemeinen die Bauchkette aus den Rumpf- 
keimen herleitet.” (No. 9, p. 259). 
In Bergh’s Fig. 23a, PI. XV., all the strata of the germ-bands 
are clearly defined; but if we compare this Fig. with Fig. 24a 
and b, the entire neuro-nephric stratum appears to be employed 
in the formation of the “ definitive epidermis.” On page 263 
the nerve-cord is said to arise beneath the “ Anlage der defini- 
tiven Rumpfepidermis.” This statement, taken in connection 
with the illustrations, of PI. XV., would lead one to suppose that 
the nerve-chain had its origin in the inner (mesoblastic) layer 
of the germ-bands. Scarcely more definite are his descriptions 
in the case of Nephelis (No. 13, p. 295, PI. XIX.). All the 
strata are clearly represented in the figures, but the neuro- 
nephric stratum is marked ep (“definitive epidermis”), and 
there is not the slightest indication of any distinction between 
neural and epidermal elements. The origin of the nervous 
system from this layer is conceded in a foot-note (p. 295), in 146 
WHITMAN 
[Vol. I. 
which my interpretation of the teloblasts is briefly noticed.1 
The same fact is more definitely stated in later papers (No. 20, 
p. 4, and No. 21, p. 408). Bergh’s interpretation of the neuro- 
nephric stratum, as a neuro-epidermal layer, will be considered 
after my own observations have been presented. 
For the latest contribution on the subject we are indebted to 
Joseph Nusbaum (Nos. 8, 22, and 23). Nusbaum has given 
a detailed account of the origin of the nerve-cord from the 
epidermal layer, thus confirming the conclusion reached by 
Kowalevsky and sustaining the position taken by Balfour (No. 
12). Nusbaum is more fortunate in his company than in his ob- 
servations, and this is almost the only reason that compels me to 
burden the reader with a review of his statements. His elaborate 
account of the mode of origin of the nerve-system from the 
epidermal layer, in view of the fact that this layer has absolutely 
nothing whatever to do with its formation, is, to say the least, a 
most singular production, and one for which it would seem, at 
first sight, very difficult to find any very satisfactory explanation. 
But it undoubtedly has an explanation, and I am under the 
disagreeable necessity of pointing it out. Nusbaum’s observa- 
tions, not only on the nerve-system, but on nearly every point 
that he has discussed, show a most evident lack of thoroughness. 
He has neglected to acquaint himself with the more important 
facts in regard to the origin and structure of the germ-bands, and 
the consequence is that he has misinterpreted and blundered at 
nearly every step. He has just as little knowledge of the anatomy 
as of the embryology of Clepsine; for he starts out under the 
persuasion that there is only one pair of testicular sacs, (No. 
22, p. 614). And yet these sacs are so large and conspicuous 
1 In justice to Bergh it should be stated that his remarks on the origin of the nerve-cord 
are intended only to serve as a preliminary account. 
(20.) Bergh, R. S. Uber die Deutung der allgemeinen Anlagen am Ei der 
Clepsinen. Zool. Anz. No. 216. 1886. 
(21.) Id. — Die Entwicklungsgeschichte der Anneliden. Kosmos. II. p. 401 
1886. 
(8.) Nusbaum, Joseph. Recherches sur l’Organogenese des Hirudinees (Clepsine 
complanata Sav.). 
Arch. Slaves de Biologie. I. fasc. 2, pp. 320-340; fasc. pp. 539-556. 1886. 
(22.) Id. — Zur Entwicklungsgeschichte der Hirudineen (Clepsine). 
Zool. Anz. VII., No. 181, p. 609. Nov., I884. 
(23.) Id. — Zur Entwicklungsgeschichte der Geschlechtsorgane der Hirudineen. 
Zool. Anz. VIII., No. 191, p. 181. Mar., 1885. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
147 
that they can easily be seen through the body-wall with the 
naked eye. What a spectacle is presented when one undertakes 
to instruct us about the genesis of organs he has never seen! 
In his second contribution (No. 23, p. 182) he discovers his 
error, but charges it to Moquin-Tandon. Has Nusbaum never 
heard of Leuckart, Leydig, or Claus, that he should go back to 
an authority of half a century ago to find out how many testiculi 
Clepsine has? But this is a trivial error in comparison with 
inaccuracies in observation and reading, such as we shall find in 
his description of the origin of the nerve-chain. 
Nusbaum (No. 8, p. 21) first attempts to explain why others 
have been less successful than himself in tracing the derivation 
of the nervous system. He discovers — so he affirms — that the 
egg-membrane is composed of two layers, the inner of which is 
provided with pores. In the course of development this porous 
layer is penetrated with vitelline granules, and, sometimes, with 
protoplasmic particles. “ D'ou il resulte que cette couche peut 
etre prise assez facilement pour Vectoderme. Alors il doit 
sembler que le systeme nerveux se forme aux depens du feuillet 
moyen, de sa couche la plus externe, aboutissant a l’ectoderme.” 
The invention of such a blunder is as preposterous as its com- 
mission is impossible. Furthermore, such a condition of the 
egg-membrane as Nusbaum has represented in Fig. 32, PI. III. 
is either artificial or altogether imaginary. Had Nusbaum be- 
gun his observations on the germ-bands at an early stage of 
their development, it would probably never have occurred 
to him to suspect his predecessors of such a stupid blunder. 
In these early stages the egg-membrane is not in contact with 
the germ-bands, and I fail to see how such a strange condition of 
the membrane could arise. 
In a brief historical review, the opinion which I formerly held 
(No. i) on the origin of the nervous system is cited as some- 
thing “ strange enough,” and as opposed to the ideas of Metsch- 
nikoff and Hoffmann. I have already made it clear that this 
opinion coincided with that of Metschnikoff and Hoffmann. 
Bergh’s conclusions on this point are not even mentioned. 
Nusbaum has described the development of the nervous sys- 
tem in his preliminary article with quite as much detail as in his 
final paper, and in nearly the same words. I shall, therefore, 
give here the original description, which runs as follows: — 148 
WHITMAN. 
[Vol. I. 
“ Wahrend der Embryo noch eine ovale Form besitzt und 
nach der Riickenseite hin stark gebogen ist, finden wir das 
diinne, einschichtige Ectoderm in dem vorderen und mittleren 
Theile des Embryo in der Mitte der Bauchseite verdickt; es ent- 
stehthier eine Schicht grosserer, zuerst runder, spater in kubische 
ubergehender Zellen. Dies ist die Anlage des Bauchnerven- 
stranges, die eine continuirliche Schicht mit dem Ectoderm bildet, 
und dicht unter der dicken derben, zweischichtigen Choriomnem- 
bran zu liegen kommt. Die Zellen dieser einschichtigen Nerven- 
systemanlage beginnen sich in der Richtung nach innen hin zu 
vermehren, und verursaclien somit die Verdickung derselben. 
Ganz unabhangig vom Bauchnervenstrange entsteht eine ahn- 
liche ectodermale Verdickung auf dem Kopfende des Embryo, 
die Anlage des Gehirnganglion bildend. . . . Die Tren- 
nung des Nervensystems vom Ectoderm findet auf folgende 
Weise Statt. Von beiden Seiten der Bauchnervensystemanlage 
bildet sich je eine diinne ectodermale Falte nach aussen hin; 
die beiden Fatten, sehr dicht dem Chorion anliegend, wen- 
den sich in der Richtung nach der Mittellinie der Bauch- 
seite des Embryo, um hier spater an einander zu stossen. Es 
bildet sich also eine Art breiter Nervenrinne, von der Seite des 
Chorion geojfnet; sie ist aber fast vollstandig flach und seicht, 
so dass ihr Boden, d. h. die zuerst gebildete Nervensystemanlage, 
dem Chorion nahe anliegt. Nach der Aneinatiderstossung der 
obengena7inten Fatten, bildet sich eine continuirliche Ectoderm- 
schicht und der Bauchnervenstrang stellt eine Art platten und 
breiten Rohrs mit einem excentrischen, engen U7id spaltformigen 
Lu7ne7i, dessen innere Wand dick ist imd de7i eige7itliche7i Bauch- 
nervenstrang vorstellt, wahrend die aussere, dem Ectoderm zu- 
gekehrte, vo7i einer einzigen Schicht platter Zelle7i gebildet ist. 
Diese Spalte sieht man noch eine Zeit lang nachher; auf spa- 
teren Stadien verschwindet sie spurlos. Es geht aus diesem 
Entwicklungsmodus hervor, dass derselbe eine nur wenig modi- 
ficirte Art der Nervensystembildung der Branchiobdella dar- 
stellt, wo sich, nach Herrn Prof. Salensky, eine grosse und tiefe 
Nervenrinne bildet, die sich in einem Nervenrohr schliesst ” (No. 
22, pp. 610-611). 
How beautifully all this chimes with the idea that a neural 
canal, comparable with that of the vertebrates, should be ex- 
pected in the annelids ! How much detail in describing some- No. 1.] 
GERM-LAYERS /AT CLEFS EVE. 
149 
thing so eminently satisfactory in theory, but without a particle 
of foundation in fact! 
Among the figures given by Nusbaum (No. 8), I find only 
one (Fig. 33, PI. III.) in which there is a median ventral 
thickening of the epidermal layer that might be mistaken for 
the basis of the nerve-chain. I have seen the same thing, 
and my first thought about it was that it represented a neural 
thickening. I shall show that this thickening is a glandular 
organ, and that the nerve-chain arises beneath it, and never 
has any connection with it. Nusbaum gives no figures in 
which such a thickening is seen at any point far behind the 
anterior ends of the germ-bands. In his Fig. 34, representing 
a transverse section of some part of the trunk of the embryo, 
there is not the slightest evidence of an epidermal thickening. 
On the contrary, the nerve-chain is here sharply marked off 
from the epidermis, although in contact with it. How does it 
happen that the nerve-chain is here farther advanced in devel- 
opment than at the anterior end? We ought, of course, to find 
the development less and less advanced as we go from the head 
towards the hind end. The broad neural groove (“ breiter Ner- 
venrinne”) shown in his Fig. 34 is purely ideal. Both Bergh 
and Hoffmann agree with me in affirming that the epidermal 
layer is here continuous, and not interrupted, as represented by 
Nusbaum. The epidermis completely covers the neuro-nephric 
stratum at a very early stage, and even advances more rapidly 
than the deeper portions of the germ-bands, meeting in the 
median ventral line, and forming a continuous layer before their 
junction, as shown in Fig. 7, PI. IV. Both the “ neural groove ” 
and the “thin ectodermal folds” are inventions, pure and simple, 
— products of a fertile imagination, which pays more respect 
to the supposed requirements of some fascinating theory than to 
the needs of thorough and accurate observation. 
Nusbaum goes completely astray in his account of the second 
layer (neuro-nephric) of the germ-bands, as will be seen from the 
following (No. 22, p. 613): “Die acht grossen Zellen, von 
Whitman ‘Neuroblasten’ genannt, die am Hinterende des Embryo 
friih auftreten und als Producte des primitiven Entoderms aufzu- 
fassen sind, erleiden folgende Veranderungen. Sie unterliegen 
einer energischen Theilung und vermehren sich fort und fort in 
der Richtung von hinten nach vorn. Auf einem friihen Stadium 150 
WHITMAN. 
[VOL. I. 
wo das Nervensystem vom Ectoderm sich noch nicht abgetrennt 
hat, findet man auf einem Querschnitt durch den hintersten 
Theil des Embryo zwei Reihen dieser Zellen, zu vier an jeder 
Seite der Bauchflache liegend; in dem etwas mehr vorderen 
Theile des Embryo sind nur deren zwei jederseits, und noch 
naher zum Vorderende beobachtet man jederseits bios eine 
einzige solche Zelle im Mesoderm, nahe der Bauchseite des 
Embryo liegend. Die Vermehrung dieser Zellen geht bis zum 
vordersten Theile des Embryo vor sich, so dass zuletzt in jedem 
Somite des Embryo jederseits je eine einzige Zelle vorkommt. 
. . . Diese grossen, in jedem Leibessegmente hervortre- 
tenden Zellen beobachtete auch Whitman, und nannte sie 
‘ Segmentzellen.’ Da er sich aber die Entwicklung des Nerven- 
systems nicht richtig vorstellte und es von den ‘ Neuroblasten,’ 
d. i. den acht grossen, oben erwahnten hinteren Zellen her- 
leitete, so bemerkte er keinen genetischen Zusammenhang 
zwischen letzteren und den Segmentzellen. Whitman vermuthete 
aber nicht unrichtig dass die Segmentzellen vielleicht an der 
Bildung der Geschlechtsorgane (Testiculi) Theil nehmen.” 
(Compare, No. 8, p. 17.) 
The elements of the two inner and principal strata of the 
germ-bands are here confounded. Nusbaum appears to be 
entirely ignorant of the existence of the two large mesoblasts 
and their relation to the germ-bands. The sexual cells 
(“segment cells”), which are derived from the mesoblasts, 
and which form a part of the third (inner) layer of the bands, 
are identified with the cells of the second (neuro-nephric) layer. 
At the hind end of the embryo, as Nusbaum affirms, are found 
four rows of these sexual cells on the ventral side of each band; 
farther forward only two rows are present; and at the anterior 
end there is only one row. Four rows reduced to one ! and no 
explanation offered. It is quite true that there are four rows of 
cells in the neuro-nephric stratum of each band; but the number 
of these cells seen in transverse section does not diminish, but 
increases from behind forward. 
It remains to notice the opinions of a few writers who, al- 
though they have made less extended studies on the develop- 
ment of the Hirudinea, or none at all, are nevertheless regarded 
as important authorities on the subject. The conclusion reached 
by Kowalevsky has had greater weight with many authors than No. 1.] 
GERM-LAYERS TN CLEPSINE. 
151 
it is really entitled to. That it is not based upon sufficiently 
thorough observations is evident enough from Kowalevsky’s 
own words. “ Die Embryonen der Hirudineen,” says Kowa- 
levsky (No. 10, pp. 1-2), “ erwiesen sich aber zu Querschnitten 
nicht ganz passend, und es gelang mir nur mit grosster Miihe, 
einige feine Querschnitte anzufertigen, auf welchen ich die 
Scheidung in Keimblatter uud die Bildung des Nervensy stems 
aus dem oberen Blatte sehen konnte, aber nicht den Keimstreifen, 
aus dem sich lediglich die Muskeln entwickeln.” 
In regard to Clepsine, Kowalevsky remarks (p. 3) : “ Da ich 
aber zur Zeit der Entwickelung derselben gerade mit den Ac- 
cipensern beschaftigt war, so bewahrte ich nur mehrere Stadien 
in schwacher Chromsaure auf; an Qnerschnitten derselben konnte 
ich mich spater nur von der Abstammung des Nervensystems 
vom oberen Blatte iiberzeugen.” 
Kowalevsky’s conclusion as to the origin of the nerve-system 
rests, then, as he himself acknowledges, on the study of a few 
eggs so imperfectly preserved and prepared that it was im- 
possible to understand the structure of the germ-bands. How 
Kowalevsky convinced himself, with such material as this, that 
the nerve-chain arises from the epidermal layer, must be left 
entirely to conjecture; for not a word of explanation is offered, 
nor a single figure given. 
Semper (No. 18), in his well-known work on “Die Ver- 
wcindtschaftsbeziehungen der gegliederten Thieve,” reviews the 
statements of Rathke, Leuckart, Kowalevsky, and Metschnikoff 
on the origin of the nerve-system in the Hirudinea, and en- 
deavors to reconcile them with his own observations on Nephe- 
lis, Nais, Chaetogaster, etc. “Nach eigenen Untersuchungen,” 
says Semper, “ kann ich fur Nephelis die Angaben Rathke’s 
bestatigen, dass die Ganglien entstehen durch Sonderung der 
medialen Parthien des Keimstreifens; eine mittlere, von diesem 
unabhangige Ectodermverdickung tritt bei dieser Gattung so we- 
nig, wie bei Chaetogaster ein" (p. 246). 
Semper, however, maintains that the nerve-chain has a double 
origin, its median portion being derived directly from the ecto- 
derm, and the lateral portions (ganglia) from the mesodermic 
layer of the germ-bands. This view enables him to explain the 
contradictory results of different writers. MetschnikofFs state- 
ments regarding Clepsine are commented on as follows (p. 178) ; 152 
WHITMAN. 
[VOL. I. 
“ Metschnikoff s inneres Blatt des Keimstreifens allein ist das 
Mesoderm, dessen Betheiligung am Aufbau des Bauchmarks er 
nicht gekannt hat; sein ausseres Blatt des Keimstreifens gehort 
dem Ectoderm an, und er hat hier ganz richtig dessen Theilnahme 
an der Bildung des Nervensystems erkannt, dagegen seine erste 
Entstehung aus dem ectoderm nicht beobachtet.” 
According to Semper, Metschnikoff and Kowalevsky are wrong 
only in supposing that the whole of the nerve-chain arises from 
the ectoderm, while Rathke was equally wrong in deriving it 
wholly from the mesoderm. Semper (p. 295) extends the idea 
of a double origin of the central nervous system to all segmented 
animals. The median part (“ centrale Ganglionzellenstrang ”), 
which is supposed to arise as an unsegmented cord, from a 
thickening of the ectoderm, is regarded as homologous with 
the spinal cord of vertebrates; the lateral ganglia, arising in 
the manner described by Rathke from the already segmented 
mesoderm, are homologized with the spinal ganglia of ver- 
tebrates.1 
Hatschek’s remarks (Nos. 24, 25) on the origin of the 
nervous system in the Hirudinea are of an incidental character, 
and are of interest only on account of the general scheme set 
up for the annelids. The ventral nerve-chain, according to 
Hatschek, arises from three rudiments, one median and two 
lateral. The median rudiment is a longitudinal infolding of 
the ectoderm along the ventral line, and is marked with a 
groove like the medullary groove of vertebrates. The lateral 
rudiments are prolongations of a pre-oral, neural plate (“ Schei- 
telplatte”), which had its origin in an unpaired thickening of 
the ectoderm. 
The “ neural groove,” which forms the most seductive feature 
of Hatschek’s scheme, has nothing whatever to do with the for- 
mation of the ventral nerve-chain, as has been clearly shown by 
Kleinenberg. In concluding his excellent review of Hatschek’s 
observations, Kleinenberg (No. 26, p. 123,) states the case thus: 
1 In a foot-note (p. 179) Semper admits that the spinal ganglia may originate in 
the neural plate, as maintained by Balfour; but, in this case, he would still maintain 
their homology with the lateral ganglia of annelids. 
(24.) Hatschek, B. Beitrage zur Entwickelungsgeschichte und Morpholo- 
gic der Anneliden. Sitzb. Akad. Wiss. in Wien. LXXIV. 1876. 
(25.) Studien liber Entwicklungsgeschichte der Anneliden. Abeiten a. a. 
zoo/. Inst, zu Wien. I. 3. 1878. No. 1.] 
GERM-LAYERS IN' CLEPSINE. 
153 
“ In ihren wesentlichen Grundlagen halte ich aber meine 
Ansichten von der Entstehung des Bauchmarkes, wie sie 1878 
fur Lumbricus und 1881 fur Lopadorhynchus entwickelt wurden, 
aufrecht, da sie durch fortgesetzte Beobachtungen an anderen 
Anneliden nicht bloss bestatigt sondern auch weiter ausgebildet 
werden konnten. Uberall entsteht der Bauchstrang unabhangig 
vom Kopfganglion aus zwei seitlichen Anlagen, ohne Betheili- 
gung einer soliden oder rohrenformigen medianen Einstiilpung 
des Ektoderms.” 
It is remarkable that so careful an investigator as Kleinenberg 
should have entirely overlooked the relation of these “ seitlichen 
Anlagen ” to the terminal neuroblasts. 
The “ neural canal ” of Branchiobdella, which Salensky (No. 
15) homologizes with the medullary canal of vertebrates, has 
also been effectually disposed of by Kleinenberg (No. 26, 
p. 127). It has just as little morphological significance as the 
“neural groove” which Nusbaum describes in Clepsine. 
Salensky discovered eight teloblasts in Branchiobdella, but 
missed their special relations and significance. 
Kleinenberg’s studies on Nephelis (No. 26, p. 129), which 
were begun some time ago, and not carried to completion, failed 
to give him the clue to the precise origin of the nerve-chain 
and its relation to the epidermal layer. The loss of the original 
epidermis, as stated by Rathke, and more fully described by 
Bergh, was also observed by Kleinenberg. 
Goette (No. 27, p. 91), adheres to the belief that the germ- 
bands are wholly mesoblastic, and discredits the testimony of 
those who derive from them the ventral chain, which, as every 
one will now admit, is ectodermic in origin.1 
(26.) Kleinenberg, N. Die Entstehung des Annelids aus der Larve von 
Lopadorhynchus. 
Zeitschr. f wiss. Zool. XLIV. I and 2. 1886. 
(27.) Goette. A. Abhandl. z. Entwicklungsgesch. der Thiere. Heft. 2. Ver- 
gleichender Theil. 1884. 
1 So strong is Goette’s faith on this point that he feels inspired to prepare for his 
heterodox brethren a very edifying homily in the form of a lengthy foot-note, warn- 
ing them of the unpleasant consequences of an “ einseitige Beurtheilung,” and admon- 
ishing them of the saving efficacy of the “ vergleichende Methode.” The counter 
homily needs no quill from the wing of the angel Gabriel. When facts conflict with 
theory we know on which side the error lies; and we have little respect for a (not 
the) “ comparative method,” which begins by denying well-authenticated facts. 154 
WHITMAN. 
[Vol. I 
Balfour based his account of the formation of the germ-layers 
in Clepsine on the observations presented in my earlier paper 
(No. 1), and offered one or two critical remarks that deserve 
notice. Referring to my statements on the origin of the nerve- 
chain from the neuroblasts, he says (No. 12, pp. 288-9) : “ Such 
a mode of origin for a ventral ganglionic chain is, so far as I 
know, without a parallel in the whole animal kingdom. 
Till more evidence is brought forward by Whitman or some other 
observer in support of the view that the so-called neuroblasts 
have any share in forming the nervous system, they must, in my 
opinion, be regarded as probably forming, in conjunction with 
the mesoblasts, two simple mesoblastic bands. Kowalevsky 
has, moreover, briefly stated that he has satisfied himself that 
the nervous system in Clepsine originates from the epiblast, — 
a statement which certainly could not be brought into harmony 
with Whitman’s account.” 
In reply to these objections the following considerations 
have been offered (No. 28, p. 392) : — 
“With reference to Clepsine, Kowalevsky remarks: ‘I pre- 
served only several stages in weak chromic acid, and from sec- 
tions of these I could only convince myself later of the origin 
of the nervous system from the upper layer.’ This is all he has 
said on this point; and I will now show that, if we do not go 
behind the verbal statement itself, it does not even require to be 
brought into harmony with my account, since it is precisely 
what I have claimed. The four rows of neuroblasts in each 
germ-band lie, at the outset, at the surface, and must therefore 
be considered a part of the epiblast, although a specialized 
part. It is simply a precocious differentiation of the edge of the 
epiblast, by which epidermal and neural elements become dis- 
tinctly marked at an unusually early stage. In the course of 
the epibolic growth of the ectoderm the epidermal portion pro- 
gresses somewhat more rapidly towards the lower pole than 
the germ-bands, and thus sweeps over the neural portion. But 
it seems to me plainly a matter of little importance whether the 
neural portion loses its surface position during the epiboly, or 
immediately after the conclusion of the concrescence of the 
(28.) Whitman, C. O. A Rare Form of the Blastoderm of the Chick, and its 
Bearing on the Question of the Formation of the Vertebrate Embryo. 
Quart. Journ. Mic. Sc., XXIII. 1883. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
155 
germ-bands; and I confess that I do not see wherein this view 
requires ‘ any special support.’ At the time Balfour penned 
the above criticism he evidently was not aware that my observa- 
tions on the origin of the nervous system in Clepsine were but 
little more than a corroboration of those of an eminent Russian 
embryologist.” 
(b) Observations on the Origin of the Nerve-chain. — The proof 
that the entire ventral nerve-chain arises as two simple lon- 
gitudinal rows of cells, and that each row is produced by the con- 
tinued proliferation of a single cell, — the neuroblast, — is to be 
obtained by the study of surface-preparations in connection with 
sections made in the planes of the three axes. Sections show 
that the fourfold striated appearance of the germ-bands is due 
to the presence of four rows of cells beneath the epidermal stra- 
tum of each band; and surface-preparations enable us to trace 
these rows forward into the special organs developed from them. 
I have been helped towards precise results by the differential ac- 
tion of the preservative fluids employed, and by natural distinc- 
tions between the neural and the nephric cell-rows. These 
distinctions are less conspicuous in the species hitherto studied 
in Europe than in the American species (C. parasita), and 
hence have been overlooked. The nephridial rows are more 
granular and stain more deeply with osmic acid than the neural 
and lateral rows. A glance at Plates IV. and V. will show how 
extremely useful such distinctions have been in the analysis of 
the neuro-nephric stratum. 
Having already briefly indicated the different layers of the 
germ-bands, it remains to consider more in detail the elements 
composing the neuro-nephric stratum. As was made clear in 
my former paper (No. i), there are exactly four rows of cells 
in this stratum in each germ-band. The outlines of these rows 
can easily be seen in germ-bands hardened in situ (Fig. i, PI. 
IV.), but better in surface-preparations which have been freed 
from the yolk (Fig. 8, PI. V.). They are most distinctly 
marked at the hind ends of the bands, but can be traced for- 
ward nearly to the cephalic lobe in many preparations. The 
reason for their becoming less and less sharply marked as we 
pass from the hind end forwards, lies in the fact that develop- 
ment is more and more advanced in this direction. Behind, the 
rows are simple (i.e., each consists of a line of single cells), and 156 
WHITMAN. 
[Vol. I. 
often separate from one another for a short distance in front of 
the teloblasts. Farther forward, each row becomes double, 
then triple or quadruple, and at the same time its boundary lines 
become less clearly defined, as shown in Figs. 8-11. Fig. 7 is 
a transverse section of the stage seen in Fig. 8, at a point where 
the bands are still separated by a narrow interval. The rows of 
cells are here simple, each showing a single cell in section. In 
another section, taken just in front of this, where the bands have 
already closed, three median cells (Fig. 6, nc) are seen in place 
of the two shown in Fig. 7. It is possible that there has been a 
duplication of cells in one of the median rows, but it is more 
probable that the section has passed through two more or less 
wedged-shaped cells belonging to the same row, but inversely 
placed, as shown in the left neural row of Fig. 11. Fig. n rep- 
resents a horizontal (frontal) section near the posterior end of an 
embryo in which the germ-bands are fully closed. All the rows 
are here simple, but towards the middle of the same embryo 
(Fig. 10) we find the neural row doubled on one side and 
tripled on the other. A little in front of the middle a still more 
advanced condition is found (Fig. 9) ; for here not only have 
the neural cells become smaller by division, but there is a plain 
differentiation into median (Long, commissures -f* median gan- 
glia) and lateral (lateral ganglia) portions. 
A more complete and instructive picture may be obtained by 
mounting the embryo entire, after stripping it from the yolk. 
In such a preparation (Fig. 8), passing from the hind end for- 
wards, we meet with successively higher stages in the develop- 
ment of the nerve-chain, beginning with two simple rows of cells 
(nc) and ending with well-defined ganglia. The neural rows 
are so clearly delimited against the darker, nephridial rows, and 
the steps in development follow in such a perfect ascending 
series, that every doubt about the conversion of these rows into 
the nerve-chain is removed. 
In Figs. 15-19, representing transverse sections from the 
posterior end of an embryo just hatched, the neural rows have 
united into a median flattened cord (nc) which shows a differ- 
entiation into median (Ic) and lateral portions, as in Fig. 9. In 
these sections, selected from a series beginning in the middle 
of one somite and extending to the middle of the next in front, 
the contrast in color between the neural and nephridial elements No. 1.] 
GERM-LAYERS IN CLEPSINE. 
157 
is well-marked. The contrast between the nephridial (nph) 
and the lateral (m') cells is equally strong, so that in this ad- 
vanced stage of development it is still easy to find all the 
derivatives of the neuro-nephric stratum, and to connect them 
with the primary cell-rows shown in Fig. 6. 
3. The Larval Gland-Cells. 
Passing now to the anterior end of the same embryo we find 
the nerve-cord presenting the same general form, with the 
median and lateral parts more clearly defined (Fig. 23). The 
median part does not yet show the double commissures. In 
this section, taken in the region marked gl in Fig. 8, we meet 
with a very interesting larval organ, consisting of numerous 
large gland-cells, each with its own duct leading to the exterior. 
These massive gland-cells lie between the sub-cesophageal 
ganglia and the epidermis, and extend over an area of greater 
breadth than the ganglia. These glands arise as a pair of thick- 
enings of the epidermal layer immediately behind the cephalic 
lobe, and appear as rounded prominences in quite an early 
stage of the germ-bands. (Fig. 1). The epidermal thickening 
is always clearly distinct from the neural cells, as may be seen 
to best advantage in longitudinal sections (Figs. 20 and 21). It 
is undoubtedly this thickening which Nusbaum has figured and 
described as the basis of the nerve-chain. Only the deeper 
cells of the thickening are destined to become gland-cells, 
and these appear to sink gradually beneath the surface, the 
ducts forming in situ rather than by subsequent outgrowths. 
A median sagittal section of this stage (Fig. 28) often shows 
only a few gland-cells compared to the number met with in 
sections passing about midway between the median ventral line 
and the side of the embryo (Fig. 29), showing that they still 
form two more or less distinct groups. 
Nusbaum (No. 8, p. 28) has described a “ provisional dorsal 
organ ” entirely different from anything I have seen. “ Chez 
l’embryon, dont le systeme nerveux est deja completement 
separe de l’ectoderme, j’ai remarque, au milieu de la paroi 
dorsale du corps, dans la troisieme partie anterieure de sa 
longueur, une couche de cellules hautes, cylindriques, de l’ecto- 
derme. . . . Les cellules ectodermiques, qui forment cette 
proeminence, emettent ensuite des fils externes, minces, tres 158 
WHITMAN. 
[Vol. I. 
longs. . . . Le r61e physiologique de ces fils consiste, 
comme il me semble, dans la fixation reciproque de jeunes in- 
dividus, tournes ordinairement l’un vers l’autre par leurs faces 
dorsales et fixes a la paroi ventrale de la mere par leurs ventouses 
anterieures. Cet organe dorsal n’existe cependant pas long- 
temps ; il disparait sans laisser de traces avant la separation des 
jeunes individus de la paroi du corps maternel.” 
Nusbaum and Hoffmann (No. 4, p. 45) have both fallen 
into the same error of supposing that the larvae attach them- 
selves to the ventral side of the parent by the oral sucker. 
The young are further, according to Nusbaum, fixed to one 
another by means of glutinous threads formed by the “ dorsal 
organ.” 
While studying the embryology of C. marginata and com- 
planata, I noticed that larvae hatched from eggs that were taken 
away from the parent and kept in a watch-glass, soon became 
attached to one another in pairs. The point of attachment, how- 
ever, was not dorsal, but ventral, just behind the part destined to 
form the oral sucker. The attachment, as I have since learned, 
is effected through an adhesive secretion of the larval glands 
above described. When the leech is allowed to remain over 
the eggs until they hatch, the larvae become fixed to its under- 
side, not by the still undeveloped oral sucker, but by the 
secretion of the post-oral, ventral glands. I have never noticed 
the “ reciprocal attachment ” by means of a “ dorsal organ ; ” but, 
without further examination, I would not venture to dispute 
Nusbaum’s statement. But the description and figures given 
certainly awaken the suspicion that the “dorsal organ” is a 
pathological formation. The larval glands which I have de- 
scribed serve only the temporary purpose of fixing the young 
to the parent leech at a time when neither sucker is sufficiently 
developed to perform this office. As soon as the posterior 
sucker becomes serviceable, it is used as an organ of attach- 
ment, and the larval glands disappear; at least, I have not been 
able to connect them with any organ in the adult leech. 
The larval organs of adhesion occupy a position which cor- 
responds to that of the Ganoid suctorial disc. The means 
of-fixation in the young fish (Amia), at least in the youngest 
larvae, is an adhesive secretion. My attention was first called to 
the secretion in Amia by Dr. Patten. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
159 
4. Primary Sense-organs of the Lip. 
At the time of hatching, long before the eyes and their serial 
homologues, the segmental sense-organs, appear, two pairs of 
sense-bulbs are found, symmetrically placed on the surface 
that is to form the margin of the lip. These organs arise as 
bulb-like thickenings of the epidermis. Figs. 12 and 13 repre- 
sent successive sections through the anterior pair of bulbs (sb'), 
in the direction shown by the arrows 12-12 and 13-13 in Fig. 
29. The fifth section behind that seen in Fig. 13 hits the 
posterior pair of bulbs (sbpassing above the super-cesopha- 
geal ganglia, as indicated by the arrow 14-14 in Fig. 29. 
These sense-bulbs are nearer together than those of the anterior 
pair, and they are a little depressed, as if there were a slight 
infolding. The scanty material at my disposal did not permit 
me to trace the history of these organs farther. I have since 
given some time to the study of the development of the eyes 
and sense-organs of the lip, in much later stages, and I have 
found that all the sense-organs of Clepsine arise in the same 
manner as these two pairs of bulbs. As the entire oesophageal 
nerve-collar is already formed, there is absolutely no ground 
for supposing that these bulbs are rudiments of the nervous 
system. The basis for the super-oesophageal ganglia (sup. ae. 
g.) is present as a distinct body of cells in the stage of Fig. 20, 
long before the appearance of the primary sense-bulbs. Even 
as early as the stage of Fig. 3, I find between the epidermal 
layer of the cephalic lobe and the primary entoderm cells a 
layer of cells which I regard as the basis of both the neural 
and the mesodermic elements of the head. The precise origin 
of this layer I have not thus far determined. 
5. The Nephridia. 
In describing the origin of the nerve-chain I have called at- 
tention to the nephridial rows of cells, two of which are found 
in each germ-band, lying between the median (neural) and the 
lateral rows (Figs. 2, 6, 7, 8, 9, 11, nph), and forming one stra- 
tum with them. These relations are shown in Diag. 9. 
In C. parasita the nephridial rows are remarkably distinct, 
owing to the contrast in color between them and the rows 
by which they are bounded. This contrast, due to the coarse WHITMAN. 
[VOL. I. 
160 
.9 
Diag. 9. — A diagrammatic surface-view of the neuro-nephric stratum at the pos- 
terior end of the 7iearly completed germ-bands, 
n, neural rows; 11b, neuroblasts; nph, nephridial rows; nphb, nephroblasts; m', 
lateral rows. . 
granules of the cells, is strengthened by the action of osmic 
acid, and thus becomes a most important aid in determining the 
fate of the cell-rows. 
Tracing the nephridial rows forward in a surface-preparation 
(Fig. 8, PI. V.), we find each represented behind by a line 
of single cells; towards the middle, they become double or 
triple, while still maintaining the same diameter; near the be- 
ginning of the anterior third, the two rows blend; and here 
outlines appear, at first shadowy, then more distinct as we 
advance, cutting the rows into quadrangular plates with 
rounded angles. The formation of the nephric plates pro- 
gresses from the cephalic end backwards, keeping exact pace 
with the metameric division of the embryo. Thus the basis is 
laid for a pair of nephridia in each somite, although only sixteen 
pairs are retained in the adult. The details of the process by 
which these plates are converted into the nephridial organs I 
have not attempted to follow. It is a point, however, which is 
worthy of a most careful study. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
161 
The series of sections shown in Figs. 15-19, beginning near 
the middle of a posterior somite, and running forwards to the 
middle of the next in advance, shows that the cells of the nephric 
plates multiply in depth as well as breadth. Fig. 18 represents 
a section on the boundary-line of two somites, in which not 
a single nephridial cell could be found. It is in this region that 
we meet with a pair of large cells (sex) lodged in the meso- 
derm. A single pair of these cells occurs in each somite, and 
their position in the walls of the septa suggests that they may 
be the mother-cells of the testicular organs. Nusbaum claims 
to have traced the development of these cells into the sexual 
organs; but he has evidently confounded the cells of the neuro- 
nephric stratum with the sexual cells. 
Returning to the nephridia, the points of chief interest in 
their development appear to be the following: — 
1. Derivation from the ectoderm. 
2. Earliest appearance in the form of simple, longitudinal 
cell-strings. 
3. Each nephridial cell-string is a product of a single ter- 
minal cell, — the nephroblast. 
As soon as I became aware of the precise origin of the ne- 
phridia, I began to question the validity of the opinion that the 
nephroblasts were ectoblastic. It was almost universally believed 
that the nephridia take their origin in mesoblastic elements. 
In view of this, I did not venture to discuss the question in my 
preliminary paper (No. 16). 
Wilson’s paper on Lumbricus settles the point, leaving little 
room for a reasonable doubt as to the ectoblastic nature of all 
the teloblasts concerned in the production of the neuro-nephric 
stratum. The establishment of this fact, taken in connection 
with recent discoveries pointing to the ectoblastic origin of 
the vertebrate segmental ducts, paves the way to a better under- 
standing of the phylogenetic derivation of these organs. 
In this connection Bergh’s discovery that the larval nephridia 
of the GnathobdellidsB arise as lateral outgrowths from the 
germ-bands is especially important. Adding this to the dis- 
covery of distinct nephridial cell-strings, we have a remarkably 
perfect picture of the more important steps in the development 
of the pronephros of Petromyzon. 162 
WHITMAN. 
[Vol. I 
6. The “Pharyngeal CleftsT 
In Fig. 1, PI. IV., are shown two remarkable grooves just in 
front of the thickened anterior ends of the germ-bands, marking 
the line of junction with the cephalic lobe. These groove- 
like formations are remarkably distinct in C. complanata, and 
are found in every species of Clepsine that I have examined. I 
have before described them as “pharyngeal clefts” (No. 1, pp. 
60-61), but I now have considerable doubt as to the correct- 
ness of this interpretation. They were not so well marked in C. 
parasita, and I have not thus far obtained the material necessary 
for a detailed study of them. 
Salensky (No. 15, p. 25) describes in Branchiobdella what he 
calls a “ bifurcation de la gouttiere medullaire,” which corre- 
sponds nearly in position and appearance with the “ pharyngeal 
clefts ” of Clepsine. 
IV. SPECIAL AND GENERAL QUESTIONS. 
1. Larval Nephridia. 
No larval nephridia have thus far been discovered in the 
Rhynchobdellidae, and the relations of these organs to the per- 
manent nephridia in the Gnathobdellidae have not yet been 
made clear. Bergh was not aware of the existence of distinct 
nephric cell-strings, and hence his descriptions and figures do 
not settle the place of origin of the larval nephridia with the 
precision that could be desired. It would seem from Bergh’s 
statements that these organs are derived from the two “ outer 
strings,” i.e.t the lateral row (in'), and the adjacent nephridial 
row in my figures. “ An den ausseren Strangen sind als schrag 
nach aussen und hinten gerichtete Zweige die Anlagen der 
Urnieren, deren Aulastoma vier Paare besitzt, sichtbar. 
Die vier Urnierenpaare entstehen somit als seitliche Sprossen 
von zwei Langsstrangen, welche letztere sich spater mit den er- 
wahnten inneren Strangen vereinigen ” (No. 29, p. 91). As 
the nature and fate of these “ longitudinal strings ” remained 
(29.) Bergh, R. S. Thatsachen aus der Entvvicklungsgeschichte der Blutegel. 
Zool. Anz. VII. No. 160, p. 90. Feb. 18, 1884. JOURNAL OF MORPHOLOGY. 
Vol. IV. No. 3. 
Table of Contents. 
S. Watase, 
Studies on Ccphalopods. 1. Cleavage of the Ovum. 
J. Playfair McMurrich. 
Contributions on the Morphology of the Actinozoa: 2. On the 
Development of the Hexactinice. 
G. Baur. On Intercalation of Vertebrce. 
William M. Wheeler. Neuroblasts in the Arthropod Embryo. 
G. Bauer. 
The Pelvis of the Testudinata; with Notes on the Evolution of 
the Pelvis in General. 
C. O. Whitman. 
Spermatophores as a Means of Hypodermic I?npregnation. 
C. O. Whitman. Description o/'Clepsine Plana. 
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GERM-LAYERS IN CLEPSINE. 
163 
unknown, Bergh (No. 30, p. 115) was led to believe that 
“ die Segmentalorgane typisch segmental, ohne die geringste 
Verbindung untereinander entstehen.” In regard to the relations 
of the larval to the permanent nephridia, Bergh says : “Bei den 
Blutegeln tritt mit grosser Klarheit hervor, dass Urnieren und 
Segmentalorgane durchans nichts miteinander zu thun haben. 
Letztere legen sich namlich (in den Rumpfkeimen) erst an 
nachdem die Urnieren sichschon langevon diesen abgelost haben. 
Ebensowenig kann von einem urspriinglichen Zusammenhang 
zwischen den einzelnen Anlagen der Segmentalorgane die Rede 
sein (No. 30, p. 113). 
If we examine Bergh’s Fig. 5, PI. XII. (No. 9), in the light 
of what we now know about the origin of the nephridia, we see 
at once that the provisional nephridia arise from one or both of 
the cell-rows, which must be identified with the nephridial rows 
of Clepsine. In this figure, the fourth nephridium (“ Urniere”) 
of the left side is represented by a single cell, which still re- 
tains its original position in the outer nephridial row. It is thus 
highly probable, if not quite certain, that both the larval and 
the permanent organs do arise from the same basis, — the 
nephridial rows. Allowing this to be the case the relations of 
the two sets of organs would be very clear. One thing is per- 
fectly certain: it can no longer be said with Bergh and 
Vejdovsky (No. 31, p. 123), that the permanent nephridia arise 
from disconnected bases or rudiments. 
The viewfirst advanced by Hatschek (No. 25), in spite of the 
theoretical objections raised by Balfour (No. 32, pp. 565-6) and 
the lack of confirmation on the part of other writers, is, after all, 
the one most easily reconciled with the results presented in this 
paper. In my opinion it not only accords better with known 
facts, but presents a more rational basis for explaining the mor- 
phogenetic relations of these organs, than the theory of discon- 
nected rudiments. I refer, of course, not to the details of the 
conditions described in Polygordius, but to general features ; such 
as, the derivation of the whole excretory system of the head and 
trunk from a common basis, and the formation of the trunk 
(30.) Bergh, R. S. Die Exkretionsorgane der Wiirmer. Kosmos. II. p. 97. 
1885. 
(31.) Vejdovsky, F. System und Morphologie der Oligochaeten. Prag, 1884. 164 
WHITMAN. 
[Vol. I. 
nephridia by metameric division of a pair of continuous longi- 
tudinal rudiments. There can be no doubt about the homol- 
ogy of the nephridial rows in Clepsine and Lumbricus with the 
longitudinal “cell-strings” of Criodrilus; and the ciliated pos- 
terior ducts, which appear to develop as sprouts from the “ head- 
kidneys ” in Polygordius, are only more highly developed forms 
of the simple nephridial rows I have described. 
Nephroblasts were not discovered in Criodrilus, but we may 
now be almost certain that they are present. We may be 
equally confident, I think, that nephroblasts, or equivalents, are 
present in Polygordius. Until they are discovered and their 
exact relations to the “head-kidneys” made out, it will be 
difficult to decide the question as to the strict identity of the lar- 
val nephridia in the Gnathobdellidae with the cephalic nephridia 
of marine annelids, such as Polygordius, Echiurus, Eupomatus 
(Serpula), etc. 
Balfour (No. 32, p. 567) was very decided in the opinion 
that “ the provisional excretory organs of the leeches cannot 
be identified with the anterior provisional organs of Polygordius 
and Echiurus.” The question now stands in a somewhat different 
light. Vejdovsky (No. 31, pp. 121-122) has discovered larval 
excretory organs in Rhynchelmis, Aeolosoma, Nais, and Chae- 
togaster, and thinks it probable that they occur in the early 
embryonic stages of all the Oligochaeta. Bergh has traced the 
development of such organs in the leeches; and his observations, 
in connection with mine, make it almost certain that both the pro- 
visional and the.permanent organs arise from the same cell-cords. 
Wilson has made the important discovery of nephric cell-cords in 
Lumbricus; and this, in connection with the wide-spread occur- 
rence of teloblasts, leaves little room to doubt that such cell- 
cords are common to all annelids. The case is made still 
stronger by the earlier observations of Hatschek on Polygordius, 
Echiurus, and Criodrilus, and by E. Meyer’s discovery (No. 33, 
p. 677) of a longitudinal canal connecting the permanent 
nephridia of Terebella (Lanice) conchilega. The general oc- 
currence of these larval organs, their relatively early origin 
(32.) Balfour, F. M. Comparative Embryology. II. 1881. 
(33.) Lang, Arnold. Die Polycladen. Fauna und Flora des Golfes von 
Neap el. Monographic XI. 1884. No. 1.] 
GERM-LAYERS IN CLEPSINE. 
165 
from, or in connection with, the nephric cell-cords, the general 
uniformity in their position, their non-metameric character, 
their atrophy and replacement by the permanent, metameric 
nephridia, appear to indicate that they all belong to one and 
the same system of organs. So far I am in accord with Bergh 
(No. 9, pp. 269-272, and No. 30, p. 116) ; but I am not of his 
opinion that this conclusion makes it impossible to homologize 
the larval with the permanent organs. 
2. Significance of Nephric Cell-Cords. 
The important bearing of the discovery of nephric cell-cords 
on the question of the derivation of the vertebrate nephric 
system has been ably presented by Wilson. Without entering 
into the discussion of this side of the question, I may say that 
I fully concur in his general views on this subject. There is 
one point only to which I will briefly call attention. If both 
the provisional and the permanent nephridia arise from the 
same cell-cords, how are we to know, from the occurrence of 
such cords, which system, if either, has been retained in the 
vertebrates? We may, as it seems to me, be quite certain about 
the homology of the nephric cell-cords, and yet be quite unable 
to decide whether one, both, or neither of the two nephridial 
systems seen in the annelids is represented in the vertebrates. 
We are not even certain that the larval nephridia represent the 
same system throughout the annelids. The mode of origin of 
the larval organs in leeches, as lateral-buds, reminds one of the 
formation of the pronephros in Petromyzon ; but the outgrowths 1 
from the “ segmental duct ” have a metameric arrangement. 
In the formation of the permanent nephridia of leeches we 
have the metameric arrangement without the lateral outgrowths, 
the entire cell-cords being cut up into consecutive cell-plates. 
The fundamental importance of homologous nephric cell-cords 
is not, however, lessened by any such difficulties in identification 
as are here presented. 
1 Scott (Morph. Jahrb. VII.) states that the pronephric funnels arise as outgrowths 
from the segmental duct, while Shipley (Quart. Jour. Mic. Sc. XXVII., Jan., 1887, 
p. 344) represents them as arising from a groove in the parietal peritoneum. As this 
groove (which is continuous with the lumen of the segmental duct) closes up, it leaves 
four or five openings which persist as the openings of the ciliated funnels. 166 
WHITMAN. 
[VOL. I. 
3. Questions Relating to the Nephridia. 
A few questions of a general nature remain to be considered. 
What morphological element (or elements) represents the primi- 
tive nephric basis? Is the non-metameric (larval) system to 
be regarded as the main stock from which the metameric (per- 
manent) system arises by a process of budding, as held by 
Hatschek? Or are the relations of the two systems better 
expressed when both are represented as buds from a common 
stock? In either case what is the primitive form of the stock 
itself? Is it a pair of simple cell-cords, or a pair of single 
cells? What was the original function of these organs? 
The answers to these questions will vary according to the 
views we entertain on the origin and significance of the meta- 
mere and its genetic relations to the head. This problem logi- 
cally takes precedence of the others; but we are not yet in a 
position to solve it, and a presentation of the leading theories 
could not well be brought within the limits of this paper. 
Besides, such work is rendered unnecessary by the excellent 
review given by Fraipont (No. 34, pp. 102-125) in the latest of 
the Naples Monographs. Bergh has recently given a compre- 
hensive and critical review of all that is known in the compara- 
tive morphology of the excretory organs of the Vermes (No. 
30 and No. 21, p. 417). 1 shall therefore limit myself here to 
a few suggestions, which appear to be warranted by the facts 
presented in this paper, when considered in the light of what 
was previously known on the same subject. 
Original Function. — Of the two functions now served by 
the nephridia, which is primary and which secondary? Bergh 
(No. 30, p. 120) holds that “ die segmentierte Leibeshohle der 
Anneliden den Hohlen der Geschlechtsfollikel der Plattwurmer 
und Nemertinen homolog ist; jede Halfte einer Segmenthohle mil 
dem sie begrenzenden Epithel entspricht cinem Geschlechtsfollikel. 
Um diesen Vergleich durchzufiihren, muss man sich vor allem 
das Verschwinden des Parenchyms bei den Anneliden verge- 
genwartigen. Dabei legen sich die Wande benachbarter Folli- 
kel (Mesodermsegmente) aneinander und in dieser Weise 
(35.) Fraipont, Julien. Polygordius. Fauna u. Flora des Golfes von Nea 
pel. Monographic XIV. 1887. No. 1.] 
GERM-LAYERS IN CLEPSLVE. 
167 
entstehen einerseits die Mesenterien, anderseits die Dissepi- 
mente.” 
In harmony herewith, it is inferred that the permanent neph- 
ridia served primarily as ducts for the escape of the sexual 
products, the excretory function having developed later. This 
view may appear plausible enough so long as we assume with 
Bergh that the larval and the permanent nephridia represent two 
unrelated systems of organs, having no connection with each 
other either ontogenetically or phylogenetically. But, let their 
homogeny be conceded, and there will be no escape from the 
conclusion that the functional relations of the permanent 
nephridia to the sexual organs, wherever such relations exist, 
have been acquired secondarily. The question then becomes 
simplified; for we have only to determine the primitive function 
of the provisional nephridia. As these organs never function 
as sexual ducts we have no reason to suppose that they have 
ever served any other purpose than that of excretory organs. 
As the permanent nephridia arose later, either directly from the 
larval organs, or, at least, from the same basis; as they exhibit 
the same general structural features; and as their appearance is 
followed by the atrophy of the larval system, there is every reason 
to believe that they assumed the work of the organs which 
they superseded. It is easy to understand how such organs 
could be pressed into the service of the sexual organs, and how 
their original function might be suppressed as the result of 
adaptation to this new work. The conversion of sexual ducts 
into excretory ones, presenting the typical structure of the 
primordial excretory organs, could not, on the other hand, be 
so readily explained. 
Original Basis. — The question of original basis, like that 
of original function, must be considered in the light of what is 
known about the development of the larval nephridia. In the 
leeches these organs appear to arise from single cells, which 
develop, by division, into simple cell-cords. This simple 
mode of development is repeated in the ontogeny of the meta- 
meric nephridia, as seen in the formation of nephridial cell- 
rows from terminal nephroblasts. Although the nephric cell- 
plates, into which the primary cell-cords are metamerically 
divided, consist of numerous cells, it is probable that each 
plate represents a simple (or double) string of cells, with its 168 
WHITMAN'. 
[Vol. I. 
coils so closely packed that the linear arrangement of the cells 
is obscured. I think this is a fair inference from the appearance 
of the nephric plate. ( Vide No. i, Fig. 92, PI. XV.) 
The larval nephridia of other annelids, so far as known, con- 
sist at most of only a few cells; and in some cases, e.g., 
Eupomatus, the duct of the fully developed organ is formed 
within a single elongated cell, stretching from the oesophagus 
back to the mesoblast of the same side. A number of spherical 
cells are found around the anterior end of this elongated cell, 
and these are regarded by Hatschek (No. 35, p. 143) as be- 
longing to the excretory organ. The entire organ arises from 
tzvo cells, one of which forms the duct, while the other splits up 
into the spherical “end-cells” (p. 134). Whether the two cells 
arise by successive divisions of the mesoblast, or by division of 
a primary nephroblast, we are not informed by Hatschek. Both 
cells are regarded as mesoblastic, but this interpretation would 
be perfectly consistent with the second mode of origin. 
Hatschek finds a pair of primary mesoblasts (“ Urmesoderm- 
zellen”). Each of these divides into two unequal parts, a 
large “ pole-cell ” and a small “ daughter-cell.” The “ pole- 
cells ” evidently correspond to the two “ mesoblasts” of Clepsine ; 
and the “ daughter-cells ” appear to me to represent nephro- 
blasts. But, if Hatschek is right in regard to the origin of these 
cells, there is one difficulty in the way of identifying the 
“daughter-cells” with the nephroblasts; for the former are 
mesoblastic, while the latter are ectoblastic. If, however, we 
examine the facts a little more closely, the objection appears 
less formidable than at first sight. In Clepsine we have seen 
one cell give rise by division to the mesoblasts, the nephro- 
blasts, and the neuroblasts. The first division separates the cell 
into a “primary mesoblast” and a “ neuro-nephroblast.” The 
point of fundamental importance for our comparison is the twin 
origin of these cells. If we call one cell mesoblastic and the 
other ectoblastic, that is a matter of interpretation, which may 
l?e justified by appearances in the one case, and contradicted 
by them in the other. The fact remains, that the genetic rela- 
tion between mesoblast and nephroblast is equally close in both 
(35.) Hatschek, B. Entwicklung der Trochophora von Eupomatus uncinatus 
Philippi (Serpula uncinata). Arbeit, a. d. zool. Inst. z. Wien. VI. H. I., p. 121. 
1885. No. 1.] 
GERM-LAYERS IN' CLEPSINE. 
169 
cases; and no artificial lines of distinction, such as we are ac- 
customed to draw between the germ-layers, can lessen its sig- 
nificance. When our definitions of the germ-layers fail us we 
must appeal to the precise genealogy of the cells. To deny the 
existence of a mesoderm is of no avail; for, with two primary 
layers, — ectoderm and entoderm, — we are just as far from 
being able to settle the question of morphological identity. 
When, as in the case under consideration, we find an organ 
arising sometimes from the ectoderm, and at other times 
from the mesoderm, we have to admit that there is no fixed 
and impassable boundary-line between these two layers; and 
that its association with this or that germ-layer is not an infal- 
lible guide to its morphological identity. 
The following view offers a fair explanation of the point in 
question: Both the mesoblasts and the nephroblasts arose 
primarily from a common ectodermic basis. The genetic rela- 
tions of the two cells have remained essentially the same; but 
the time of their differentiation as distinct cells varies. If the 
division takes place within the ectoderm, then each makes its 
exit from the original seat separately and independently of the 
other; if, on the other hand, the division is delayed until after 
the separation from the ectoderm is accomplished, then the 
nephroblast appears to arise from the same source as the meso- 
blastic bands, and thus to form a part of these bands. The dif- 
ferences noted between Eupomatus and Clepsine may be recon- 
ciled in this way. The conflicting accounts given of the origin 
of the vertebrate “ segmental duct ” admit of a similar ex- 
planation. 
There are, then, some very positive indications that the larval 
nephridium consisted, originally, of a single cell; and the gen- 
eral occurrence of nephroblasts, as the basis of both systems of 
organs, is in favor of this view.1 
4. The Origin of the Epidermis. 
Bergh has shown that it is necessary to distinguish between 
the larval and the definitive epidermis of the Gnathobdellidse. 
11 am reminded of the opinion long ago expressed by Leuckart (No. 17, pp. 698- 
699), that the teloblasts of Clepsine (“ Colossale Zellen ”) represent “ Urnieren 
Leuckart supposed, however, that they were provided with ducts, and that they were 
functionally active. 170 
WHITMAN. 
[Vol. I. 
The larval epidermis arises in precisely the same manner as the 
epidermis of Clepsine ; it is lost during larval life, and replaced 
by the “definitive1’ epidermis, which is an entirely separate and 
independent formation from the neuro-nephric stratum, having 
absolutely no direct genetic relations with the original epi- 
dermis. The origin of the definitive epidermis, as described 
by Bergh, has no parallel in other animals, and it is clearly im- 
possible to reconcile it with my observations on the fate of the 
neuro-nephric stratum. The mode of reconciliation suggested 
by Bergh (No. 20, p. 6), according to which the epidermis of 
Clepsine is the homologue, not of the larval, but of the defini- 
tive epidermis of the Gnathobdellidae, must be set aside as en- 
tirely incompatible with the facts presented in this paper. 
There can be no doubt about the accuracy of Bergh’s obser- 
vations on the loss of the larval epidermis; but his theory of 
the origin of the definitive epidermis I am not able to accept 
on the evidence adduced. I have shown that three of the four 
rows of cells constituting the neuro-nephric stratum of each 
band are employed in the formation of the nerve-chain and the 
nephridia. I have not been able to satisfy myself fully as to 
the fate of the fourth or lateral row; but I have followed it far 
enough to ascertain that it has nothing whatever to do with the 
formation of the epidermis. The epidermis is perfectly distinct 
at every stage from the neuro-nephric stratum, and I cannot 
discover the shadow of a reason for thinking that it ever re- 
ceives any contributfons from this stratum. 
In this connection I must mention one fact which links the 
teloblasts with the epidermis. I have shown that the neuroblast 
is the twin cell of the median nephroblast. The mother-cell 
(No. 1, PI. XII., Fig. 35, x3), before dividing into these two cells, 
produces a small median cell (;r4), which, together with its 
homotype of the opposite side, is converted directly into true 
epidermal cells. This pair of epidermal cells (x4) is a constant 
and striking feature of the stage referred to. This is the nearest 
point of connection between the epidermic layer and the neuro- 
nephric stratum. But I venture to say that no one acquainted 
with the development of Clepsine would risk the suggestion that 
the whole epidermis is derived from this median pair of cells. 
Kleinenberg (No. 26, p. 129) confirms Bergh’s statement as 
to the loss of the original epidermis in Nephelis, but raises objec- No. 1.] 
GERM-LAYERS IN CLEPSINE. 
171 
tions to his idea of the origin of the definitive epidermis. Klein- 
enberg adds (p. 130), that in all Polychaeta, whose develop- 
ment is known to him, the epidermis of the larva is replaced by 
the definitive, outer epithelium of the annelid; but this takes 
place through a process of transformation, which has its point 
of departure in the larval epidermis itself, and only in a few 
cases are the parts of the old ectoderm actually thrown off. 
Is it not possible that the permanent epidermis in Aulostoma 
and Nephelis has its origin in the larval epidermis? Such an 
origin would accord perfectly with what takes place in other 
annelids, and remove the apparent discrepancies in develop- 
ment between the Rhynchobdellidae and the Gnathobdellidae. 
Bergh’s figures appear to lend no support to the suggestion; 
but important points in the history of this neuro-nephric stra- 
tum have escaped his attention, and a reexamination is required 
in order to settle them. If it turns out that this stratum gives 
origin to the nephridia and nerve-chain, as in Clepsine, the con- 
formity in development between the two classes of leeches will 
be settled beyond a doubt. 
5. Significance of the Teloblasts. 
The teloblasts form one of the most remarkable features of 
annelid development. They represent specialized centres of 
proliferation, with most marvellous powers of assimilation and 
reproduction. Their occurrence in worms, molluscs, and verte- 
brates (only mesoblastic teloblasts have thus far been dis- 
covered outside the annelids), in larval as well as in foetal types 
of development, makes it sufficiently evident that they are not 
to be regarded as an accidental phenomenon without morpho- 
logical significance. 
The embryos of all bilateral animals, from the worms up to 
the vertebrates, lengthen by cell-proliferation at the posterior end. 
The question arises, Is this proliferating power invariably local- 
ized in special cells or groups of cells? It is generally believed 
that the posterior end of the embryo represents a mass of indiffer- 
ent, non-specialized elements. It is supposed that here histolog- 
ical differentiation has its vanishing point; that here the germ- 
layers blend in a common basis. 
The case of Lumbricus (vide Wilson’s paper) shows us that 172 
WHITMAN. 
[Vol. I. 
the teloblasts may differ so little in size from the cells which 
they produce that their terminal position is about the only 
means of distinguishing them. This accounts for their having 
been overlooked by such embryologists as Kowalevsky, Klein- 
enberg, and Hatschek. In the Hirudinea we see the different 
kinds of teloblasts of each band represented either by one or 
two cells. Wilson informs me that in one species of Lumbricus 
he finds only one nephroblast on each side; in another species, 
two, as in Clepsine. 
We do not know to what extent this variation in number may 
be carried; but it adds another difficulty of recognition, which 
might easily become insuperable. Instead of only one or two 
teloblasts of a given kind, there may be many, all taking equal 
shares in a common work, or correlative parts of, a complex 
work. Some such condition may be supposed to exist in the 
higher bilateral animals. 
We already have sufficient grounds for regarding the telo- 
blasts as an archaic feature of development. Obviously they 
do not represent primitive organs, but the undeveloped, embry- 
ological bases of such organs. They constitute the trunk-bud, 
and are thus the primary seat of all the truly metameric ele- 
ments of the animal. Primarily they represented, as we have 
reason to suppose, the bases of non-metameric organs, in which 
the regenerative power was, or became, preeminent. 
6. The Foetal and the Larval Type of Development. 
The relations of the foetal and the larval types of develop- 
ment have never been made clear by those who hold that the 
latter represents, approximately, the ancestral line of develop- 
ment. Some have maintained that the phylogenetic history of 
the annelid is retraced in larval metamorphoses; while others 
have denied any such morphogenetic significance to the larva, 
claiming that it is a secondary form reached through adaptive 
changes which have been called forth by its pelagic mode of life. 
Balfour has given us a broad and comprehensive discussion 
of the nature, origin, and affinities of larval forms, and has con- 
sidered, in a general way, the nature and extent of the second- 
ary changes likely to occur in the foetal or the larval state. 
According to Balfour (No. 32, p. “the relative chances No. 1.] 
GERM-LAYERS IN CLEPSINE. 
173 
of the ancestral history being preserved in the foetus or the 
larva may be summed up in the following way: There is a 
greater chance of the ancestral history being lost in forms 
which develop in the egg; and of its being masked in those 
which are hatched as larvae.” 
Balfour’s phylogenetic conclusions were based on a compari- 
son of the various larval forms with one another. No attempt 
was made to identify larval with foetal features of development, 
and to verify in this way deductions based on the occurrence of 
similar larvae in different groups. It cannot be denied, how- 
ever, that in this direction lies a crucial test of our theories re- 
specting larval forms. As long as it remains impossible to find 
a parallel in fundamental features between the foetus and the 
larva, so long will it be impossible to decide how much is ances- 
tral and how much adaptive in the larva. 
In spite of volumes devoted to the discussion of the subject 
the larva of Polygordius still remains a morphological puzzle. 
After an extended, critical analysis of the leading theories re- 
lating to this larva, Fraipont closes his magnificent monograph 
on Polygordius with the following confession: “ It is not yet 
possible, in the present state of our knowledge, to determine 
what is the morphological significance of the larva of Poly- 
gordius, the Trochophora (Trochosphere) of the annelids.” 
In the history of the teloblasts we find a satisfactory basis 
for the direct comparison of the foetal with the larval course of 
development. What, then, is the foetal Trochosphere? and of 
what importance is it to our theoretical conceptions of the an- 
nelid embryo? Does it throw any light on the structure of the 
ancestral Trochosphere, — the Trochozoon? and does it assist 
us to a better understanding of the nature and extent of the 
abbreviations and modifications represented in direct develop- 
ment? 
For the purpose I have in view Clepsine furnishes an excel- 
lent example of the direct type of annelid development, while 
Polygordius affords a well-known example of the larval type. 
These forms may, therefore, serve as points of departure for the 
few suggestions to be offered here. The development of the 
Trochosphere of Polygordius is very imperfectly known, but the 
gap is now bridged by Hatschek’s studies on Eupomatus 
(No. 35). 174 
WHITMAN1 
[VOL. I. 
We have seen that radial symmetry, so far as outward appear- 
ances go, is preserved in the egg of Clepsine until the eight-cell 
stage is reached; and that bilateral symmetry attains its full- 
est expression through the cleavage of the posterior macromere, 
which ends in the establishment of ten teloblasts. In Eupoma- 
tus we reach the same important stage of development by the time 
the blastopore has been reduced, by closure advancing from be- 
hind forward, to a small pore, — the future mouth. The radial 
Gastrula has passed into the bilateral, embryonic stage of the 
larva. The teloblasts are represented by a pair of mesoblasts 
(“ pole-cells ” ) and a pair of nephroblasts. The neuroblasts are 
not distinguishable, which may be explained on the supposition 
that they still lie in the ectoderm, and present no conspicuous 
differential characters. A prae-oral band of cilia (prototroch, 
Kleinenberg), occupying an equatorial position with respect to 
the primary axis, is already present, and the apex of the prae- 
oral lobe (Scheitelfeld) bears a few long cilia. With the ex- 
ception of the ring and the tuft of cilia, the organs of the larva 
are as yet undeveloped, and exist only in the form of more or 
less definite rudiments. The embryonic Trochospliere, as we may 
call this stage, is represented in Hatschek’s Figs. 25 and 26, 
PI. XI. 
In Clepsine, as I have said, the formation of the teloblasts, 
as the closing act of cleavage, brings us to the stage which cor- 
responds most nearly to the embryonic Trochospliere. For the 
sake of distinction this stage (Diag. 4) may be named the fxtal 
Trochospliere. 
In both forms we meet with the same fundamental features, 
and the differences are precisely such as general principles 
would lead us to anticipate. Ciliated organs of locomotion, 
specially adapted to the needs of a roving, larval life, but with- 
out functional importance for the foetus, are not developed in 
Clepsine. As the foetal Trochosphere is supplied with a large 
stock of food-material, ready for absorption without the aid of a 
digestive system, there is no necessity for an early development 
of the mouth and gastric cavity, such as must exist in the case 
of the embryonic Trochosphere; and, accordingly, we find the 
larval development far in advance of the foetal in these particu- 
lars. In respect to the trunk-bud (teloblasts), the case is re- 
versed ; for in the larval embryo the differentiation of the bud No. r.] 
GERM-LAYERS IN CLEPSINE. 
175 
is incomplete, and its development is retarded in the interest of 
the Trochosphere proper; while in the fcetal form, the trocho- 
spheral development is abbreviated for the sake of a more per- 
fect bud with accelerated development. 
Among the more important differences remaining to be noticed 
are those which have been brought about under the influence 
of the food-yolk. The process of gastrulation, the form of the 
blastopore and its relations to the mouth, have been very pro- 
foundly modified in this way. The trunk-bud of the fcetal 
Trochosphere has been carried far from its original, post-oral 
position; and, as the result of this displacement, we see the 
halves (germ-bands) of the trunk, which develop side by side 
as a unit in the larva, formed separately, and carried over the 
massive sphere of yolk in such a manner as to meet along the 
median ventral line. This whole process of circumcrescence 
and concrescence has arisen secondarily, in adaptation to fcetal 
conditions that do not exist in the larval form. The blastopore, 
if we include the space traversed in the closure of the germ- 
bands, has been stretched out of all proportion to its original 
dimensions, so that it no longer represents the primitive Gas- 
trula-mouth, but merely a secondary prolongation of it back- 
wards along the whole ventral line of the body. In the embry- 
onic Trochosphere we find the blastopore already closed before 
the trunk-bud begins to develop; hence the line of closure 
(“ Gastrula-raphe ”) is limited to the ventral line of the Trocho- 
sphere. As the metameric body-region is not yet developed, it 
is evident that the posterior limit of the primitive blastopore 
falls within the non-metameric region, from which the head- 
segment of the adult animal is formed. This region is repre- 
sented in Clepsine by the cephalic lobe, and the primitive blas- 
topore is not, strictly speaking, represented at all. The most 
that can be said is that its position is sometimes indicated by a 
linear depression (No. I, p. 55), extending from the mouth to 
the hind edge of the cephalic lobe. The line of junction of the 
germ-bands becomes continuous with the post-oral groove; but 
the two things have nothing further in common, and they are 
as distinct in meaning as in mode of origin. What is usually 
called the blastopore is therefore a secondary opening resulting 
from the mechanical separation of the halves of the trunk-bud, 
and its closure is simply a restoration of original conditions. 176 
WHITMAN. 
[Vol. I. 
This closure advances from before, backwards, following the 
direction of the bud-development. The closure of the primi- 
tive blastopore, on the contrary, progresses in the opposite 
direction, and represents, not a restoration, but a reconstruction. 
In the embryonic Trochosphere the anterior remnant of the 
blastopore persists as the mouth, while in the foetal Trocho- 
sphere, where the primitive blastopore never comes to develop- 
ment, the mouth appears to form as a secondary perforation. 
Grant that teloblasts exist in both the larval and the foetal 
form, and that the conception of them as a trunk-bud is 
correct, and I see no escape from the above view of the blasto- 
pore. The larval form has the primitive blastopore with typical 
relations to the mouth and to the non-metameric portion of the 
animal; the foetal form has lost the primitive blastopore, and 
acquired a secondary one, which may be regarded as a posterior 
extension of the original opening along an entirely new region 
— the metameric trunk-region. 
The primitive Gastrula stage is passed long before the estab- 
lishment of the first metamere; the secondary Gastrula is the 
primitive one extended, and so retarded in development that the 
process of gastrulation is prolonged through the whole forma- 
tive period of the embryo. 
The occurrence of the Trochosphere, its origin from a typical 
invaginate Gastrula, the persistent relations of the blastopore to 
the mouth, and the presence of a teloblastic trunk-bud, all 
appear to me to support the views developed in the foregoing 
comparison. 
It is conceivable that the Gastrula from which the Trocho- 
sphere arises represents not a primitive, but a derived form, 
which has been much reduced in extent through a retarded 
development of the trunk. The objections to this view are 
numerous, and so obvious that they need not be enumerated. 
The comparison above made between the foetal and the larval 
Trochosphere has important bearings on the interpretation of 
the blastopore in higher forms, on the concrescence theory 
of the formation of the vertebrate embryo, and on some recent 
theories of the origin of metameric segmentation, — bearings 
which cannot be considered in extenso within the limits of the 
present paper. I may say, however, that I see no reason for 
abandoning the so-called theory of concrescent growth. In all No. 1.] 
GERM-LAYERS IN CLEPSINE. 
177 
cases where separate germ-bands are formed concrescence must 
be conceded. The formation of the vertebrate embryo may be 
easily regarded as a modification of the same process, and a 
more rational view has not yet been propounded, so far as I am 
able to judge. Concrescence seems, indeed, to be the very 
essence of Sedgwick’s theory (No. 36) of the origin of the 
mouth and anus, and yet he rejects this view as applied to 
vertebrates. 
It will be obvious to the reader of the foregoing pages that I 
regard as untenable those theories according to which the 
somites of segmented animals are derived from gut-pouches. 
It is not the archenteron, nor yet the mesenteron, in which me- 
tamerism first exhibits itself. 
36. Sedgwigk, Adam. Origin of Metameric Segmentation. Quart. Journ. 
Mic. Sc., XXIV. Jan., 1884. 
BIBLIOGRAPHY. 
12. Balfour, F. M. A preliminary Account of the Development of 
Elasmobranch Fishes. Quart. Journ. Mic. Sc., XXII., p. 323. 1874. 
32. Id. —A Treatise on Comparative Embryology, Vol. II. 1884. 
9. Bergh, R. S. Die Metamorphose von Aulastoma gulo. Arbeiten a 
d. zool.-zoot. Inst, in Wurzburg., VII., H. 3, p. 231. 1885. 
13. Id.—Uber die Metamorphose von Nephelis. Zeitschr. f. wiss. Zool., 
XLI., H. 2, p. 284. 1884. 
20. Id. — Uber die Deutung der allgemeinen Anlagen am Ei der Clepsinen 
und der Kieferegel. Zool. Anz., No. 216. 1886. 
21. Id. — Die Entwicklungsgeschichte der Anneliden. Kosmos, II., p. 401. 
1886. 
29. Id. — Thatsachen aus der Entwicklungsgeschichte der Blutegel. Zool. 
Anz., VII., No. 160, p. 90. Feb. 18, 1884. 
30. Id. — Die Excretionsorgane der Wurmer. Kosmos, II., p. 97. 1885. 
11. Butschli, O. Entwicklungsgeschichtliche Beitrage. Zeitschr. f wiss. 
Zool., XXIX., p. 239. 1877. 
34. Fraipont, Julien. Polygordius. Fauna u. Flora des Golfes von 
Neafel. Monographic XIV. 1887. 
19. Fillippi, F. de. Sopra l’Anatomia e lo Sviluppo delle Clepsine. 
Pavia, 1839. 
27. Goette, A. Abhandl. z. Entwicklungsgesch. der Thiere, H. 2. 
Vergleichender Theil. 1884. 
2. Grube, A. E. Untersuchungen uber die Entwicklung der Clepsinen. 
Konigsberg, 1844. 
24. Hatschek, B. Beitrage zur Entwickelungsgeschichte und Morpholo- 
gic der Anneliden. Sitzb. Akad. Wiss. in Wien, LXXIV. 1876. WHITMAN. 
[VOL. I. 
25. Id. — Studien fiber Entwicklungsgeschichte der Anneliden. Arbeiten 
a. d. zool. hist, zu Wien, I., 3. 1878. 
35. Id.—Entwicklung der Trochophora von Eupomatus uncinatus Philippi 
(Serpula uncinata). Arbeiten a. d. zool. Inst. z. Wien, VI., H. 1. 
1885. 
4. Hoffman, C. K. Zur Entwickelungsgeschichte der Clepsinen. Nieder- 
landisches Archiv. fur Zoologie, IV., H. 1., p. 31. 1877. 
6. Id.—Untersuchungen fiber den Bau und die Entwickelungsgeschichte 
der Hirudineen. Haarlem, 1880. 
26. Kleinenberg, N. Die Entstehung des Annelids aus der Larve von 
Lopadorhynchus. Zeitsclir. f. wiss. Zool., XLIV., 1 & 2. 1886. 
10. Kowalevsky, A. Embryologische Studien an Wfirmern und Arthro- 
poden. Mem. de I Acad. Imp. de Sc. de St.-Petersbourg, XVI., 
No. 12. 1871. 
33. Lang, Arnold. Die Polycladen. Fauna und Flora des Golfes von 
Neapel. Monographie XI. 1884. 
17. Leuckart, R. Die Menschlichen Parasiten, I. 1863. 
7. Metschnikoff, Elias. Beitrage zur Entwickelungsgeschichte eini- 
ger niederen Thiere. Vorlaufige Mittheilung. Melanges Biologiques, 
VII., p. 671. 1871. 
8. Nusbaum, Joseph. Recherches sur l’organogen&se des Hirudin^es 
(Clepsine complanata Sav.). Archives slaves de Biologie, I., Fasc. 
2, pp. 320-340, and Fasc. 3, pp. 539-556. 1886. Published separate. 
22. Id.—Zur Entwicklungsgeschichte der Hirudineen (Clepsine). Zool. 
Anz., VII., No. 181, p. 609. Nov., 1884. 
23. Id.—Zur Entwicklungsgeschichte der Geschlechtsorgane der Hiru- 
dineen. Zool. Anz., VIII., No. 191, p. 181. Mar., 1885. 
3. Rathke, H. and Leuckart, R. Beitrage zur Entwicklungsgeschichte 
der Hirudineen. Leipzig, 1862. 
5. Robin, C. Memoire sur le Ddveloppement Embryogenique des Hiru- 
Paris, 1875. 
15. Salensky, W. Ddveloppement de Branchiobdella. Arch, de Biol., 
VI., Fasc. I., p. 1. 1885. 
36. Sedgwick, Adam. Origin of Metameric Segmentation. £>uart. Journ. 
Mic. Sc., XXIV. Jan., 1884. 
18. Semper, C. Die Verwandtschaftsbeziehungen der gegliederten Thiere- 
Arb. a. d. zool.-zoot. Inst, in Wurzburg, III. 1876. 
15. Vejdovsky, Fr. Die Embryonalentwicklung von Rhynchelmis (Eu- 
axes). Vorlaufige Bemerkungen. Sitz.-Ber. d. k. bohm. Ges. d. 
Wiss. 1886. 
31. Id. — System und Morphologie der Oligochaeten. Prag, 1884. 
1. Whitman, C. O. Embryology of Clepsine. Quart. Journ. Mic. Sci., 
XVIII. 1878. 
16. Id.—The Germ-Layers of Clepsine. Zool. Anz., No. 218. 1886. 
28. Id. — A Rare Form of the Blastoderm of the Chick, and its Bearing on 
the Question of the Formation of the Vertebrate Embryo. Quart. 
Journ. Mic. Sc., XXIII. 1883. 179 
GERM-LAYERS IN CLEPSINE. 
[No. I. 
EXPLANATION OF PLATE IV. 
Reference Letters. 
a, left macromere. 
b, right macromere. 
c, anterior and median macromere. 
cl, cephalic lobe. 
ec, ectoderm. 
en, entoderm. 
enp, entoplast. 
ep, epidermal layer. 
m, mesoderm. 
m’, lateral cell-row. 
nb, teloblasts of the neuronephric 
stratum. 
nc, nerve-cord, neural cell-rows, neu- 
roblast. 
nph, nephridial rows. 
p, posterior end of cephalic lobe. 
x, right mesoblast. 
xy, left mesoblast. 
y, yolk. 
Fig. i. C. complanata from Naples. Surface view of germ-bands in an 
equatorial position. The white blotches in the yolk are entoplasts (enp.) 
X 120. 
Fig. 2. C. complanata (Naples). Transverse section near the middle of the 
egg, in a little earlier stage. The contrast in color between the superficial and 
the deeper cells of the ectoderm has been made too great by the lithographer. 
X 120. 
Fig. 3. C. complanata (Naples). Median sagittal section of the same 
stage. X 120. 
Fig. 4. Cephalic lobe and underlying yolk, from the same series of sec- 
tions, but a little to one side of the median piane. X 280. 
Fig. 5. An obliquely horizontal section of the same stage, at the level of 
the arrow 5-5 in Fig. 3. X 120. 
Fig. 6 C. parasita (Cambridge, Mass.). Transverse section of the em- 
bryo, when the germ-bands are a little more than two-thirds closed. Section 
is just in front of the unclosed portion of the bands. X 280. 
Fig. 7. From the same series, iust behind the point of closure. Jo urn. Morjih. Vol.I. 
PL nr 
W&kitsicai del 
Lith.Anst.vWemerdWbtter.Frcmkfkrt a/M. WHITMAN. 
[VOL. I. 
EXPLANATION OF PLATE V. 
Reference Letters. 
c, coelom. 
en, entoderm. 
ep, epidermis. 
g, lateral ganglia. 
gl, larval glands. 
Ic, longitudinal commissures. 
m, mesoderm. 
ml, lateral cell-row. 
nc, nerve-cord — neural rows. 
nph, nephridial rows — nephridia. 
oe.c, oesophageal collar. 
s, septum. 
sb,' anterior pair of sense-bulbs. 
sb," posterior pair of sense-bulbs. 
sex, sexual cells. 
sp, somatopleure. 
spp, splanchnopleure. 
st, mouth. 
st.d, stomodaeum. 
y, yolk. 
Fig. 8. C. parasita. Surface view of the embryo when the germ-bands 
are about two-thirds closed. 
Figs. 9-1 i. Three portions of a horizontal (frontal) section of the embryo, 
after the complete closure of the germ-bands, showing different stages in the 
development of the neuronephric stratum. Fig. 9, near the end of the first 
third; Fig. 10, near the beginning of the last third; and Fig. 11, very near the 
hind end. X 280. 
Figs. 12-14. From a series of transverse sections of the head at time of 
hatching. Fig. 12 hits the dorsal edge of the stomodseal ingrowth, and 
grazes the anterior pair of sense-bulbs (sb") ; Fig. 13 is the next section above, 
taking in a strip of the oesophageal collar and the centre of the sense-bulbs; 
Fig. 14 is the fifth section (= .oo75mm) above Fig. 13, and hits the posterior pair 
of sense-bulbs (sb") ; y = the line of the yolk. The position of these sections 
is shown by the arrows in Fig. 29. X 280. 
Figs. 15-19. Selected from a series of transverse sections through the hind 
end of the embryo at time of hatching, showing an early stage in the develop- 
ment of the nerve-chain and the nephridia. The sections run from near the 
middle of one somite to the middle of the next in front. Fig. 15 gives the 
first section; Fig. 16, the second; Fig. 17, the fourth; Fig. 18, the fifth; and 
Fig. 19, the seventh. 
Fig. 18 is taken on the boundary line of the two somites. X 280. Journ. Morjih. Vol.I. 
PL V 
Whitman del. 
LithAnst.v.Wema'dWinter,Frankfurt 182 
WHITMAN. 
[VOL. I 
EXPLANATION OF PLATE VI. 
Reference Letters. 
c, coelom. 
d, ducts of larval gland-cells. 
en, entoderm. 
enp, entoplasts. 
ep, epidermis, 
g, ganglia. 
gl, larval gland-cells. 
m, mesoderm. 
tic, nerve-cord. 
nl, neurilem. 
p, proboscis. 
s, septa. 
sb', anterior pair of sense-bulbs. 
sgl, salivary glands. 
sp, somatopleure. 
spp, splanchnopleure. 
st.d, stomodamm. 
sub.K.g, subaesophageal ganglia. 
sup.ce.g, superoesophageal ganglia. 
tr, transverse nerve-fibres. 
y, yolk. 
Fig. 20. C. parasita. Median sagittal section of an embryo in which the 
germ-bands are one-half closed. X 280. 
Fig. 21. Similar section of a stage in which the germ-bands are nearly 
closed. X 280. 
Fig. 22. Middle region of ventral side (from the same section), showing 
entoplasts in the periphery of the yolk. X 280. 
Fig. 23. Transverse section through the region of the larval gland-cells at 
time of hatching. X 280. 
Fig. 24. Near the middle of the ventral side. From a sagittal section fol- 
lowing that seen in Fig. 28. 
Fig. 25. C. marginata. Nine days old. Longitudinal section of one of 
the anterior gastric diverticula. The dorsal half alone is shown. X 280. 
Fig. 26. C. parasita. Same series as Fig. 28. Middle region of ventral 
side, showing the septa and an early stage of the entoderm. X 465. 
Fig. 27. From the same section, nearer the hind end. X 465. 
Fig. 28. Median sagittal section at time of hatching. X 280. 
Fig. 29. Sixth section from that shown in Fig. 28. X 280. Jonrn Mornh. Vol.I. 
PL . VI. 
i Whitman del 
LithAnst.vWemerMrterJranWwrt a'M