THE BODY CAVITY. 



625 



sp.c 



obliterated : they are dealt with in a separate chapter. The 

 ventral part of the primitive cavity alone constitutes the 

 permanent body cavity. 



The primitive body cavity in the lower Vertebrata is at first 

 continued forwards into the region of the head, but on the 

 formation of the visceral clefts the cephalic section of the body 

 cavity becomes divided into a series of separate compartments. 

 Subsequently these sections of the body cavity become oblite- 

 rated ; and, since their walls give rise to muscles, they may 

 probably be looked upon as equivalent to the dorsal sections of 

 the body cavity in the trunk, and will be treated of in connection 

 with the muscular system. 



As a result of its mode of origin the body cavity in the 

 trunk is at first divided into two 

 lateral halves ; and part of the meso- 

 blast lining it soon becomes distin- 

 guished as a special layer of epithe- 

 lium, known as the peritoneal epithe- 

 lium, of which the part bounding the 

 outer wall forms the somatic layer, 

 and that bounding the inner wall the 

 splanchnic layer. Between the two 

 splanchnic layers is placed the gut. 

 On the ventral side, in the region of 

 the permanent gut, the two halves 

 of the body cavity soon coalesce, 

 the septum between them becoming 

 absorbed, and the splanchnic layers 

 of epithelium of the two sides uniting 

 at the ventral side of the gut, and 

 the somatic layers at the median 

 ventral line of the body wall (fig. 



.0 



-sp.v. 



In the lower Vertebrata the body 

 cavity is originally present even in 

 the post-anal region of the trunk, but 

 usually atrophies early, frequently 

 before the two halves coalesce. 



On the dorsal side of the gut the 



B. ill. 



FIG. 351. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM EM- 

 BRYO SLIGHTLY YOUNGER THAN 



18 F. 



sp.c. spinal canal ; IV. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 x. sub-notochordal rod ; ao. aorta ; 

 mp. muscle-plate ; mp' '. inner layer 

 of muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body ; st. segmental 

 tube; sd. segmental duct; sp.v. 

 spiral valve ; v. subintestinal vein ; 

 P.O. primitive generative cells. 



40 



