THE METAMERISM OF THE DORSAL AND THE 
VENTRAL LONGITUDINAL MUSCLES 
OF THE TELEOSTS. 
ROSS GRANVILLE HARRISON. 
Under this head are to be considered, the mm. supra-carinales ; the infra- 
carinales including the retractor ischii; the rectus abdominis, and the coraco- 
hyoideus. These muscles are all slender bands of longitudinal fibres, are 
paired in origin, and generally lie close to the mid line, in the case of the 
supra-carinales at the dorsal edge of the lateral body muscles or myotomes, 
and in the case of the others at the ventral edge. Of the dorsal muscles 
there are two ; one extends from the occipital to the first ray-support of the 
dorsal fin; the other, from the posterior ray-support of this fin to the tail. 
The analogous muscles of the ventral side connect the hard parts of the 
extremities in a similar way. Am infra-carinal extends from the anal fin 
to the tail; a similar muscle originates from the posterior portion of the 
pelvis, and running posteriorly, encircles the anus, and is attached to the 
first ray-support of the anal fin; this muscle has been called the retractor 
ischii. The anterior end of the pelvic bone is connected with the shoulder 
girdle by a similar muscle. This naturally varies according as the position 
of the ventral fin varies with respect to the pectoral. It varies also in the 
degree of its independence from the lateral body muscles. In the salmon 
it is a quite independent band of muscle fibres, and its position in the adult 
and its mode of development justify us in homologizing it with the rectus 
abdominis profundus of the amphibia. [See Maurer, Der Avfbau und die 
Entwicklung der ventralen liumpfmuskulatur bei den Urodelen Amphibien. 
Morph. Jahrb., Bd. 18.] Anterior to the shoulder girdle, this muscle is con- 
tinued forwards as the coraco-hyoideus, which is attached to the base of 
the hyoid arch. 
These facts regarding the myology of the adult teleosts have long been 
known. The points which lam here able to contribute to our knowledge 
of their development in the salmon may not prove uninteresting. 
Excepting the coraco-hyoideus,* the anterior of the supra-carinales is the 
first to make its appearance. In the region of the anterior myotomes of 
embryos in the dorsal fins of which muscle buds have just appeared, there 
arise aggregations of mesoderm cells, one on each side of the mid line, 
dorsal and median to the dorsal edge of the lateral muscles. The cells 
which make up these masses are of doubtful origin; they may wander there 
♦ The development of this muscle is described in my paper on the fins. singly from the dorsal edge of the myotomes, where the cells are still un- 
differentiated, or they may he mesenchyme cells which have collected at 
these points. At first no traces of segmentation are present. These cell 
aggregations extend slowly backwards until they nearly reach the dorsal 
fin, and finally they become attached to its anterior ray-support by means 
of a tendon of some length. Long before these loose strands of cells have 
reached the dorsal fin, the anterior ones begin to differentiate into muscle- 
cells. Segmentation now arises in the young muscle, which becomes 
divided into a series of small segments connected by tendons. This seg- 
mentation arises independently of the metamerism of the body and bears no 
constant relation to it; the segments are from one and a half times to twice 
as long as the myotomes. Those of the two sides do not correspond with 
one another but show a tendency to alternation. 
Just after the completion of the anterior muscle the posterior makes its 
appearance in a similar way and develops in like manner except that it is 
formed almost simultaneously throughout its entire length. The ventral 
muscles or infra-carinales also arise similarly except that the cells giving rise 
to them show at first an undoubted connection with the ventral edge of the 
myotomes. These muscles are all segmented in a manner similar to the 
dorsal ones. 
The development of the rectus abdominis differs in important details from 
that of the other ventral muscles, although, like them, it arises from the un- 
differentiated ventral edge of the myotomes. In young salmon in which the 
anterior supra-carinal muscle has reached the dorsal fin, but which still 
possess a large yolk, the myotomes posterior to the pectoral fin have grown 
in the body wall, out over the yolk-sac for some distance, forming a rather 
thin lamina of muscle {venlraler Urwirbelfortsatz of Maurer). The inter- 
muscular septa run obliquely forwards and ventrally. The periphery of 
this muscle lamella is composed of a band of undifferentiated cells. 
Beginning with the myotome just in front of the anterior end of the 
definitive pelvis, this band of cells soon separates from the myotomes. This 
separation, gradually extending forwards, there arises a strand of undiffer- 
entiated cells which is independent of the lateral muscles. A good while 
before the anterior portion of this embryonic muscle has acquired its inde- 
pendence, the posterior portion begins to differentiate into muscle fibres, 
and like the other muscles becomes segmented, but not correspondingly with 
the metamerism of the body. The anterior portion also differentiates into 
muscle cells, and not yet having lost its connection with the processes from 
the myotomes, septa are formed here at corresponding points, and, unlike the 
posterior portion of the muscle, the anterior portion corresponds in its 
metamerism with the rest of the body. The factor which renders this dif- 
ference in the two portions of one and the same muscle possible is that, 
while in general all differentiations appear first in the anterior metameres 
and extend gradually backwards, this muscle becomes independent at its 
posterior end first; its tissue, however, differentiates in accordance with the 
general law and the consequence is, that when differentiation begins, con- 
ditions not being alike throughout the whole extent of the muscle, a difference 
arises in the metamerism of the two parts. In conclusion then, it is seen that certain muscles, although segmented, 
have acquired their segmentation independently of the general metamerism 
of the body, and that even the same muscle may in different portions exhibit 
different relations between its own segmentation and that of the myotomes. 
The segmentation of muscles, as observed in the adult, therefore, may or may 
not have been secondarily acquired, and in view of this, it would seem that 
observations on the adult anatomy alone are not to be relied upon for 
determining the number of myotomes which originally took part in the 
formation of the muscle.