ELASMOBRANCHII. 49 



portion ; thereby the upper segmented part of the body cavity 

 becomes isolated, and separated from the lower and unseg- 

 mented part. As a consequence of this change the vertebral 

 plate comes to consist of a series of rectangular bodies, the 

 mesoblastic somites, each composed of two layers, a somatic 

 and a splanchnic, between which is the cavity originally continu- 

 ous with the body cavity (fig. 23, mp}. The splanchnic layer of 

 the plates buds off cells to form the rudiments of the vertebral 

 bodies which are at first segmented in the same planes as the 

 mesoblastic somites (fig. 22, Vr\ The plates themselves re- 

 main as the muscle-plates (mp}, and give rise to the whole of the 

 voluntary muscular system of the body. Between the vertebral 

 and lateral plates there is left a connecting isthmus, with a 

 narrow prolongation of the body cavity (fig. 23 B, st], which 

 gives rise (as described in a special chapter) to the segmental 

 tubes and to other parts of the excretory system. 



In the meantime the lateral 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. In the tail however the plates do not unite 

 ventrally till somewhat later, and their contained cavities remain 

 distinct till eventually obliterated. 



At first the pericardial cavity is quite continuous with the 

 body cavity ; but it eventually becomes separated from the 

 body cavity by the attachment of the liver to the abdominal 

 wall, and by a horizontal septum in which run the two ductus 

 Cuvieri (fig. 23 A, sv}. Two perforations in this septum (fig. 23 

 A) leave the cavities in permanent communication. 



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

 including special organs or the vascular system) are as 

 follows : 



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 inter-muscular connective tissue. 



(4) Part of the peritoneal epithelium. 

 From the splanchnic layer are formed 



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



B. III. 4 



