DEVELOPMENT OF ELASMOBRANCH FISHES. 113 



posed of two layers, a somatic and a splanchnic, between which 

 is the cavity originally continuous with the body-cavity. The 

 splanchnic layer of the plates buds off cells to form the rudi- 

 ments of the vertebral bodies which are originally segmented 

 in the same planes as the proto vertebrae. The plates them- 

 selves remain as the muscle-plates and develop a special layer 

 of muscle {m p) in their splanchnic layer. 



In the meantime the parietal plates of the two sides unite 

 ventrally throughout the intestinal and cardiac regions of the 

 body, and the two primitively isolated cavities contained in them 

 coalesce. Posteriorly however the plates do not unite ventrally, 

 and their contained cavities remain distinct. 



At first the pericardial cavity is quite continuous with the 

 body-cavity ; but by the close of the period included in the 

 present chapter it becomes separated from the body-cavity by a 

 septum in front of the liver, which is however pierced dorsally 

 by two narrow channels. 



The parts derived from the two layers of the mesoblast (not 

 including special organs or the vascular system) are as follow : — 



From the somatic layer are formed 



(1) A considerable part of the voluntary muscular system 



of the body. 



(2) The dermis. 



(3) A large part of the intermuscular connective tissue. 



(4) Part of the peritoneal epithelium. 



From the splanchnic layer are formed 



(1) A great part of the voluntary muscular system. 



(2) Part of the intermuscular connective tissue (?). 



(3) The axial skeleton. 



(4) The muscular and connective-tissue wall of the 



alimentary tract. 



(5) A great part of the peritoneal epithelium. 



General Considerations. In the history which has just been 

 given of the development of the mesoblast, there are several 

 points which appear to me to throw light upon the primitive 

 origin of that layer. Before entering into these it is however 

 necessary to institute a comparison between the history of the 

 B. 8 



