Fig. 380. Transverse section through the anterior part of the trunk of an embryo of Scyllium slightly older than fig. 29 B.
The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length; whereas they join the spinal cord halfway between two posterior roots.
sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; dn. dorsally directed nerve springing from posterior root; mp. muscle-plate; mp´. part of muscle-plate already converted into muscles; m.pl. part of muscle-plate which gives rise to the muscles of the limbs; nl. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric) duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of hepatic duct with duodenum; umc. umbilical canal.
The muscles of the limbs. The limb muscles are formed in Elasmobranchii, coincidently with the cartilaginous skeleton, as two bands of longitudinal fibres on the dorsal and ventral surfaces of the limbs ([fig. 346]). The cells, from which these muscles originate, are derived from the muscle-plates. When the ends of the muscle-plates reach the level of the limbs they bend outwards and enter the tissue of the limbs ([fig. 380]). Small portions of several muscle-plates (m.pl) come in this way to be situated within the limbs, and are very soon segmented off from the remainder of the muscle-plates. The portions of the muscle-plates thus introduced soon lose their original distinctness. There can however be but little doubt that they supply the tissue for the muscles of the limbs. The muscle-plates themselves, after giving off buds to the limbs, grow downwards, and soon cease to shew any trace of having given off these buds.
In addition to the longitudinal muscles of the trunk just described, which are generally characteristic of Fishes, there is found in Amphioxus a peculiar transverse abdominal muscle, extending from the mouth to the abdominal pore, the origin of which has not been made out.
It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish-like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. That these muscle-plates give rise to the main muscular system of the trunk, at any rate to the episkeletal muscles of Huxley, is practically certain, but the details of the process have not been made out.
In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk. In the tail of the Amniotic Vertebrata the primitive arrangement is also more or less retained, and the same holds good for the dorso-lateral trunk muscles of the Lacertilia. In the other Amniota and the Anura the dorso-lateral muscles have become divided up into a series of separate muscles, which are arranged in two main layers. It is probable that the intercostal muscles belong to the same group as the dorso-lateral muscles.
The abdominal muscles of the trunk, even in the lowest Amphibia, exhibit a division into several layers. The recti abdominis are the least altered part of this system, and usually retain indications of the primitive intermuscular septa, which in many Amphibia and Lacertilia are also to some extent preserved in the other abdominal muscles.
In the Amniotic Vertebrates there is formed underneath the vertebral column and the transverse processes a system of muscles, forming part of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked out, but on the whole I am inclined to believe that it is derived from the muscle-plates. Kölliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin.
Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out.
It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kölliker however believes that they have an independent origin.
The limb-muscles, both extrinsic and intrinsic, as may be concluded from their development in Elasmobranchii, are derived from the muscle-plates. Kleinenberg found in Lacertilia a growth of the muscle-plates into the limbs, and in Amphibia Götte finds that the outer layer of the muscle-plates gives rise to the muscles of the limbs.