In the higher Vertebrata on the other hand the entrance of the muscle-plates into the limbs has not been made out (Kölliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscle-plates.
The Somites and muscular system of the head.
The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head. In the adult condition no true Vertebrate exhibits indications of such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent to the somites of the trunk: they have been frequently alluded to in the previous chapters of this volume. These structures have been most fully worked out in Elasmobranchii.
The mesoblast in Elasmobranch embryos becomes first split into somatic and splanchnic layers in the region of the head; and between these layers there are formed two cavities, one on each side, which end in front opposite the blind anterior extremity of the alimentary canal; and are continuous behind with the general body-cavity ([fig. 20] A, vp). I propose calling them the head-cavities. The cavities of the two sides have no communication with each other.
Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft; and at a later period it becomes, owing to the formation of a second cleft, divided into three sections: (1) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft; (3) a section behind the first branchial cleft.
Fig. 381. Transverse section through the front part of the head of a young Pristiurus embryo.
The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the premandibular and mandibular head-cavities 1pp and 2pp, etc. The section is moreover somewhat oblique from side to side.
fb. fore-brain; l. lens of eye; m. mouth; pt. upper end of mouth, forming pituitary involution; 1ao. mandibular aortic arch; 1pp. and 2pp. first and second head-cavities; 1vc. first visceral cleft; V. fifth nerve; aun. auditory nerve; VII. seventh nerve; aa. dorsal aorta; acv. anterior cardinal vein; ch. notochord.
The front section of the head-cavity grows forward, and soon becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution. The posterior part lies completely within the mandibular arch.
As the rudiments of the successive visceral clefts are formed, the posterior part of the head-cavity becomes divided into successive sections, there being one section for each arch. Thus the whole head-cavity becomes on each side divided into (1) a premandibular section; (2) a mandibular section (vide fig. 29 A, pp); (3) a hyoid section; (4) sections in each of the branchial arches.