WITH COMPLIMENTS OF THE AUTHOR 
Placental Development 
BY 
HENRY O. MARCY, A.M., M.D., LL.D., 
OP BOSTON, XJ. S. A., 
President of the Section of Gynaecology, Ninth International Medical Congress; late President of the American 
Academy of Medicine ; Member of the British Medical Association ; Member of the Massachu- 
setts Medical Society; Member Boston Gynaecological Society; Corresponding 
Member of the Medico-Chirurgical Society of Bologna, 
Italy; late Surgeon U. S. Army, etc. * 
[Reprinted from the “Transactions of the Ninth International Medical Congress,” Yol. ll.] 
PRESS OF WM. F. FELL A CO., 1220-24 6ANSOM ST., PHILA., PA. PLACENTAL DEVELOPMENT. 
DEYELOPPEMENT PLACENTAL. 
ENTWICKELUNG DER PLACENTA. 
HENRY 0. MARCY, M.D., 
Of Boston, Mass. 
Four years ago, Italy mourned the loss of one of her most brilliant sons, Count 
Ercolani, of Bologna, a man, like Germany’s great scientist, Virchow, alike celebrated 
as a leader in the great political struggles for the elevation of his people and an enthu- 
siastic devotee of science. Educated in the school of the great Antonio Alessandrini, 
whose learned diligence he early imitated, in the most lamentable poverty of means, 
he contributed to the foundation of those monuments of marvelous industry, the 
museums of Comparative Anatomy and Veterinary Pathological Anatomy, of Bologna. 
The high renown which he had acquired in the sciences and his profound learning pro- 
cured him many honors. He attained the highest offices of Bologna’s most famous 
university, being several times President of the Medical Faculty and twice Hector of 
the University. Aided by the government, he was enabled to erect new buildings and 
furnish the School of Veterinary Medicine with modern appliances for successfully 
carrying on original investigations in the department of comparative anatomy and 
physiology. Although widely known by his many original contributions to science, he 
is especially deserving of and receives world-wide repute for his long-continued investi- 
gations of the placental development in vertebrates. 
More than ten years since, there first came to my notice an unpretending volume, 
published in Africa, a translation in French, by Dr. Andreini, of a certain portion-of Pro- 
fessor Ercolani’s investigations, which as a prize essay had received an award from the 
Academy of Sciences at Paris. In our Harvard Library, I found in the Transactions 
of the Academy of Sciences at Bologna the original papers with illustrations, published 
from time to time, having been presented by him as contributions to the Academy. 
Convinced of the singular value, as well as the originality of his work, I collated and 
presented to the English reading public the first edition, which included all his 
anatomical researches, up to the date of the publication, the last chapters of which 
were written specially for this English edition. In 1880 was published a second and 
enlarged edition, which included also Professor Ercolani’s researches upon the patho- 
logical conditions of placental development, with a careful analytical review of the 
whole subject, written while yet suffering from the dire malady which speedily there- 
after terminated his life. During all this period, I myself have carefully studied the 
changes incident to the uterus in the reproductive state in woman, and, as far as oppor- 
tunity has been afforded me, comparative studies in animals. lam thereby convinced 
more than ever of the correctness of Professor Ercolani’s teaching, and hold in admira- 
tion the remarkable ability and skill by which he arrived at conclusions so far-reaching, 
and demonstrated with singular clearness nature’s uniform law of the unity of 
anatomical type in a simple and fundamental plan of embryonic nutrition and 
development. 
The present occasion offers opportunity only to review very briefly some of the more 
important deductions to be made therefrom. In our own profession the study of repro- 
duction has been chiefly limited to the human species, and nothing is more complex or 
confusing, veritably a labyrinthian riddle, than the fully developed human placenta. 
The uterine mucosa had been studied with some care by many investigators, in both 
woman and in the lower animals, for generations. To Malpighi, of the fifteenth century, we date the first reliable demonstrations upon the mucous membrane in woman and in 
some species of the lower animals. He described the openings of the utricular glands 
into the uterine cavity, and observed that these glands, at least in the cow, increased 
in size during pregnancy. When we consider the lack of optical instruments in his 
day, his observations are worthy of special comment. Little was added to this 
anatomical knowledge by scientists until within the present generation.» 
Professor Ercolani, as the trained comparative anatomist, early devoted his attention 
to a careful microscopic differentiation of the mucous membrane with its glandular 
structure, in all the various species which he was enabled to bring' under his observa- 
tion. This naturally led to the investigation of the more complicated question of their 
function during pregnancy, and thus, little by little, was brought int*> differential 
study placentation. 
The placenta is ordinarily subdivided into diffused, multiple and single. Perhaps 
the simplest form of the first is found in the mare. Over the whole maternal portion 
of the uterus, in this animal, there are developed a series of secretipg glands of follicu- 
lar character, and into these it is easy to trace the villi of the foetal p'o’rtion of the 
placenta. A foetal villus is little more than a vascular loop covered with epithelium. 
The glandular follicle is equally simple in anatomical cOpstruation, and also lined with 
epithelium ; the one, a villus of secretion, the other of absorption.-* This is the simplest 
possible type of the double structure of the placenta. " 
The multiple placenta of the cow offers the simplest form of, this kind of placenta, 
common to ruminants. The glandular maternal organ is hei4 mo<Jjfied, although it 
does not lose its elementary form of a simple follicle. The modification consists chiefly 
in the uterine follicles being placed parallel to the surface, and sometimes superim- 
posed upon each other, instead of, as in the diffused placenta, being disseminated 
vertically over the whole internal surface of the uterus. 
In the dog and cat the follicles are extraordinarily elongated into tubular glands, 
as it were, which are closely packed against the foetal villi. * 
In woman, all that relates to the form itself of a glandular follicle, is completely 
lost, but the fundamental parts of a secreting organ, that is to say, the walls and cells, 
in a word, the gland and its secretion, are persistent. « 
The lesson taught by comparative anatomy lies in the following of these changes in 
the more simple form of placentation, and observing that in each the nutrition of the 
foetus is provided for upon this simple plan. Having once determined with accuracy 
these facts, we are then, for the first time, prepared to devote ourselves intelligently to 
the discussion of the placental development in woman. It is sufficient here to make 
the simple statement of that which maybe conceded as an accepted fact, that in woman 
the mucous membrane is reduced to a simple layer of epithelial cells, and that in 
impregnation there is a proliferation and destruction of these cells over the entire 
surface of the uterus. This destructive process of the epithelium on the internal 
surface of the uterus is, in all cases, indispensable, because this is what facilitates the 
setting up of the neo-formative changes from which will result the maternal portion of 
the placenta. This denudation of the internal surface of the uterus teaches that the 
formation of the decidua and the placenta is due, neither to a tumefaction nor to a 
t. ansformation of the anatomical elements preexistent at the time of conception in the 
uterine mucous membrane. 
The neo-formative process of the maternal portion of the placenta consists in the 
production of new vessels which are distinguished from the ordinary uterine vessels ; 
first, the arterial as well as the venous vessels, have only a simple endothelial wall ; 
second, on the external surface of their wall is elaborated a layer, more or less thick, 
of special cells, not separable from the wall of the vessel. These are the so-called 
decidual or placental cells. That the foetal portion of the placenta is itself due to a neo-formative process has 
not been and cannot be questioned. It is from the constant relation established 
between these two parts of neo-formation that the placenta is developed. The manner 
in which this relation is established, gives rise to the different forms of placenta known 
in the mammalia. Except as viewed by Ercolani, the opinions which have been enter- 
tained, concerning the origin of the cells which enter so largely into the formation of 
the maternal part of the placenta, have been exceedingly vague and uncertain. Pro- 
fessor Turner, although concisely affirming, with Owen, that without decidua there is 
no formation of placenta, does not touch the important question of the origin of the 
decidua. 
Professor Kblliker confines himself to the remark that the decidua is a transforma- 
tion of the uterine mucous membrane, and not a new membrane, or the product of 
an exudation, as was once believed. It may be observed, however, that Professor 
Kblliker does not attempt to show which are the elements in the mucous membrane 
that compose the decidua, nor by what means they are transformed, although such an 
investigation should be of the highest interest to him, since the glandular follicles of 
new formation, as demonstrated by Prof. Ercolani, and largely confirmed by Turner, 
were only declared by him to be tumefactions of the preexisting uterine mucous mem- 
brane, formed during pregnancy and disappearing after delivery. 
The elementary and typical form of the two parts constituting the placenta in the 
mammifera, is not imaginary, but is demonstrated in its simplicity, by careful obser- 
vation. It is easily recognized in the foetal portion in the villi of the chorion, in the 
simpler forms of diffused placentae, and at the beginning of development in the human 
species. In the maternal portion, the simple elementary form is found to he devel- 
oped, and maintained through the whole period of gestation, in the uterus of certain 
viviparous fishes. The manner in which the relation between the two parts is estab- 
lished may be by simple proximity, contact, or by intimate cohesion. When the rela- 
tion is that of simple nearness, the maternal portion of the placenta manifestly presents 
the form of a glandular organ, and has its limitation by the repetition of secretory villi 
upon the inner surface of the uterus, which, uniting with each other in various ways, 
give rise to the formation of crypts, or glandular follicles, single or compound, into 
which enter the absorbent villi of the chorion. 
When the relation is more intimate and an adherence takes place between the two 
parts before mentioned, as in the single placenta, the glandular character is concealed 
by the fact of the adhesion, but the fundamental condition remains constant. The 
contact in this case is direct, between the vessel of the absorbent villus and the epi- 
thelium of the secreting villus, which is never lost. Only two very simple changes 
occur in the fundamental parts of the placenta when single, and they are the factors 
of the manifold differences which are observed. First, the loss of the epithelium of 
the absorbent villus, which is not important, since there is established direct contact of 
the vessel of the foetal villus with the secretory epithelium of the maternal villus, and 
this fact is constant. Second, the dilatation, or ectasia of the vessel in the maternal 
villus, and this fact is remarkable only in the placenta of the quadrumana and in 
woman. 
The ectasia in the maternal vessels, already shown under a rudimentary form in 
the placenta of certain mammals, had been indicated by Eschricht and Turner, as rep- 
resenting the large lacunas which are observed in the placenta of some of the quadru- 
mana, and in that of the human species. But these supposed lacunae have been the 
chief, if not the only stumbling-block to the exact knowledge of the structure of the 
human placenta, although anatomists were aware that ectasia takes place in the pla- 
cental vessels in their phases of development. 
It is known that, in the earliest period of pregnancy, the placenta in the human species is represented by a layer formed of a rather compact cellular mass called 
decidual serotina, or placental decidua, in the midst of which runs a network of capil- 
lary vessels. From the surface of the chorion which is in contact with this layer, the 
villi are formed, at first simple, which penetrate among the cells of the serotina. For a 
time the relation between these two parts of the placenta is not so close that they can- 
not he separated. It is known, moreover, that, with the progress of the development, 
the vessels of the primary capillary network become ectasic, and the primary simple 
chorial villi proliferate and become arborescent. If we could loosen and skillfully 
separate without tearing, the compact mass formed by the cells of the decidua sero- 
tina, before the vessels become ectasic, we should obtain exactly what comparative 
anatomy has very clearly shown in the single placenta of certain mammals, the dog 
and cat, for example, namely, a large meshed network of placental vessels everywhere 
surrounded by a cellular layer, placental cells. 
We may easily demonstrate, in the placenta of early abortions, the gradual develop- 
ment of the lacunae by ectasia, from the beginning, in the network of the capillaries 
amidst the mass of the placental cells, while the villi of the chorion are still quite 
simple. 
Eobin described this condition with great accuracy. The formation of the lacunae 
precedes, therefore, the formation of the tufts of the villi, and cannot be an effect, 
since they are observed before the existence of the cause assigned by Prof. Kolliker. 
Still further and more conclusive evidence upon the formation of the lacunae, inde- 
pendent of the presence of the foetal villi, may be found in the structure of the so- 
called uterine decidua in cases of extra-uterine pregnancy, and there is found the exact 
and isolated anatomical structure of the maternal portion of the placenta, in which exist 
lacunae, through ectasia of the vessels, without any trace of the foetal villi. Kolliker 
himself teaches that the short utero-placental arteries, when they penetrate into the 
placenta, lose their distinctive anatomical characteristics, that is, they no longer have 
muscular fibres or elastic elements; and their whole wall is formed by an endothelial 
layer, covered with a thin sheath of connective tissue, which disappears and blends 
with the decidua serotina. He further states “ that the veins are no longer to be dis- 
tinguished from the arteries, and all traces of both are lost in the interior of the 
placenta where alone the large lacunae are found.” 
In 1876, De Sinfety called attention to an important demonstration in its bearing 
upon the above view. He noticed that the cells of the decidua in woman form a cir- 
cular sheath about the placental vessels. However, if the facts observed by him in 
the completely developed human placenta harmonize with those already noticed early 
in its development, when there is seen a minute network of capillaries that become 
ectasic in the midst of the cells of the decidua or placental serotina, we shall be con- 
vinced that it is not the walls of the utero-placental vessels which are lost in these 
cells, as was indicated by Kolliker, but that so enormous a dilatation has taken place 
as to render it difficult to perceive the endothelium and to separate it from the pla- 
cental cells that are elaborated by their external walls. 
The chorial villi in woman, while yet existing in its proliferation around the entire 
sac, are always covered with an epithelium of their own. The chorial villi, away from 
the placental site, in the atrophic state of advanced foetal development, may still be 
seen covered with epithelium, although shrunken and atrophied. 
But in the intimate cohesion with the maternal villi in the developing placenta, the 
epithelium covering the chorial villus is lost. This may be easily demonstrated by 
observations upon the placenta in abortions, where the death of the foetus has for some 
time preceded the separation of the placenta. Here the foetal villus is shriveled and 
shrunken, and is easily defined as distinct from the maternal villus with its layer of 
actively growing placental cells. The belief that the lacunae are really large cavities, as they seem to he microscopic- 
ally, and not the maternal vessels greatly dilated, is still generally taught, and through 
this belief two other deceptive appearances are accepted as actual truths, namely, that 
the chorial villi float in the maternal blood, and that the epithelium covering them 
appertains to the foetus instead of to the mother. Setting aside these fundamental 
errors, many facts already observed by able anatomists in the human placenta which 
have remained doubtful or were wrongly interpreted, now receive, under the teaching 
of Ercolani, a clear and precise explanation. 
The belief that the villi in the placenta of woman float in the blood of the lacunse, 
generated the physiological error that the nutrition of the foetus, not only in the human 
species, but in all mammals, took place through an osmotic exchange of the two bloods, 
although in the case of the diffused placenta the facts openly contradict such an asser- 
tion. Beside, in all cases where the placenta is single, the vessel of the absorbent 
villus (foetal) never comes in contact either with the blood or with the wall of the 
maternal vessel. There is always interposed between the walls of the two vessels, and, 
consequently, of the two bloods, a cellular layer wThich is the epithelium of the maternal 
villus, and that this is secretory is confirmed by the obvious glandular appearance 
which is observed in many animals in the maternal portion of the placenta when it has 
the diffused form. 
Briefly reviewed, these changes take place in woman during pregnancy. The uterine 
mucous membrane, which consists of a simple layer of the epithelium closely attached 
to the subjacent muscular wall, disappears. It is replaced by a layer of decidual cells, 
proliferated from the vascular network of the uterine wall. The utricular glands are 
not destroyed, but, on the contrary, are increased in size. The constant secretion 
therefrom forms openings through the decidua vera, which may easily be traced by 
the unaided vision, as a sieve-like perforation. By the proliferation of the serotinal 
cells at the placental site, these glands are, by the pressure from their own obstructed 
secretion, dilated and altered, and this forms the strata, called by Kolliker the non- 
caducous portion of the placenta. Traces of these glands may be observed, even to the 
termination of pregnancy, but none whatever are found in the caducous stratum of the 
placenta, as defined by Kolliker, where it would be expected that they would be found, 
if, as claimed by him, the placenta is a transformation of the uterine mucosa. 
All anatomists agree that the stratum where the placenta is formed is developed 
from a mass of cellular or decidual elements, traversed hy a vascular network, with 
endothelial walls only during the first months of pregnancy, and that among these 
elements the fcetal villi are buried. During the first three months of pregnancy, the 
fcetal villi can be separated from the maternal portion without a laceration of the 
latter, in the same way as in the mammiferous uon-deciduates. 
The second period of development, by the universal consent of anatomists, begins 
with the establishing of two facts, the rapid and exuberant proliferation of branches 
from the trunk of the fcetal villi, and the ectasic process in the network of the maternal 
vessels. These conditions taking place contemporaneously, in a limited and circum- 
scribed space, it must necessarily follow, as the result of simple physical and mechanical 
law, that the branches of the proliferating villi should press against the endothelial 
walls of the vessels, which are, at the same time, thinned and dilated. The ultimate 
result is, that the walls of the vessels, at first bent in simply toward the internal 
cavity of the dilating vessel, must, as the process of aneurism becomes more and more 
pronounced, completely invest the villus, aided also by the tension exercised upon the 
walls of the vessel by the rapidly proliferating branches of the villi. 
Weber and Yirchow observed this condition in abortions, describing the tufts of 
the villi as making a complete hernia in the maternal vessel. Resulting therefrom 
the epithelial covering of the chorial villus disappears. Intimate union with the intro- fleeted wall of the maternal vessel, lined with its layer of decidual cells, ensues, and 
the picture of the villi swimming in the lacunae is complete. 
Professor Ercolani sums up his conclusions as follows ; In all the vertebrates the 
nutritive material, which is to serve for the growth of the foetus, is provided by the 
mother. In mammals it is supplied by the maternal portion of the placenta gradually, as 
the foetus develops. In the oviparous vertebrates, the material, in the quantity necessary 
for the development of the foetus, is emitted in a mass from the mother in the egg. 
In the mammiferous, as in the oviparous animals, the absorbent or foetal part does not 
change, and it js by means of an absorbent villus, more or less complicated, that the 
material elaborated by the mother is conveyed to the foetus. There is, therefore, but 
one law, a physiological modality, that governs the nutrition of the foetus in vertebrates. 
There is also always preserved an unfailing typical unity of anatomical structure 
in the placenta, in the different classes and species of mammals, and the variety and 
multiplicity of forms depend only on a few very simple modifications, observable in 
the two fundamental parts, the foetal and the maternal, which constitute the placenta 
in the mammifera and in the human species. 
Prof. Simpson said the obstetricians were much indebted to Dr, Marcy for 
coming from the Section over which he was presiding,* to give them such a lucid and 
instructive exposition of the recent observations on the structure and development of 
the placenta. He heartily endorsed Dr. Marcy’s admiration of the illustrious professor 
of Bologna, whom he (Prof Simpson) had the satisfaction of seeing at work in his 
laboratory, in 1872, and whose researches he followed with greater confidence, from 
the impression he had then gained of Ercolani’s earnestness and honesty of pur- 
pose. The method Dr. Marcy has adopted of expounding his views as to the 
placental relations was a happy one, and, indeed, the only satisfactory one, proceed- 
ing from the simplest to the most complicated plans of placentation. The problem 
solved in the construction of the placenta, was how to bring foetal and maternal 
blood vessels into such relations as would admit of interchange of effete and nutri- 
tive materials between the one system and the other. In the simplest forms, as in 
the ridged placenta of the sow, or the diffused placenta of the mare, the whole 
surface of the chorion was vascular and applied to a corresponding hypermmic sur- 
face of the uterine mucosa, slight foetal ridges, or projections, fitting into shallow 
furrows or depressions, with such slender connection that when the ovum was 
expelled, not even the epithelial surface of the uterus was affected. In the poli- 
cotyledonary placenta of the ruminants, specially enlarged groups of foetal villi on 
limited portions of the chorionic surface dipped down into specially developed crypts 
in limited areas of the uterine mucosa, and when the ovum was expelled, the foetal 
villi were often found to have carried off some of the uterine epithelium attached to 
their extremities. In the still more limited zonary placenta of the carnivora, the fusion 
became still more intimate, so that on expulsion of the ovum some of the sub-epithelial 
decidual tissue was carried off by the foetal villi, without, however, causing any neces- 
sary injury to the maternal blood vessels. Where the placental site became still more 
distinctly limited, as in the case of the two-toed sloth, Sir William Turner, to whose 
researches Dr. Marcy had referred, and whom he (Prof. Simpson) was proud to claim 
as his colleague and friend, had made the interesting observation that the capillaries in 
the maternal mucosa had become distended at parts presenting an aneurismal or 
DISCUSSION. 
* Gynaecology. varicose appearance. This helped to the understanding of the changes that took 
place in the capillaries of the human serotina, which probably passed through a 
similar stage before becoming developed into the placental sinuses, which Weber had 
described as “colossal capillaries.” In the human placenta, the fusion of chori- 
onic and mucosal tissues had become so intimate and intricate, that when the ovum 
was thrown off, the chorion carried with it the greater part of the thickness of the 
uterine mucous membrane, leaving only a thin layer corresponding to the blind 
extremities of the follicles attached to the subjacent muscular wall. The exact condi- 
tion of the capillary arrangement in the placenta, it was right to remember, could not 
yet be regarded as definitively settled. In a recent work published by the lamented 
Schroeder, in conjunction with some of his pupils, one of these had an article deny- 
ing the existence of the cavernous spaces usually supposed to exist in the placenta, 
and further observations must be made at various stages of development before a 
true anatomy of the placental vessels can be arrived at. As regards the simple 
layer of epithelial cells, which most observers now recognize as the only intermediary 
between the maternal and foetal blood, he (Prof. Simpson) had, at one time, followed 
Virchow in regarding them as of foetal origin. The more recent observations, 
however, of Turner and others, had convinced him that Dr. Marcy was correct in 
following Ercolani, who was the first definitely to declare them to be maternal in 
their origin.