ELASMOBRANCHII. 



41 



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. 

 The dermis. 



A large part of the intermuscular connective tissue. 

 Part of the peritoneal epithelium. 

 From the splanchnic layer are formed 



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

 Part of the intermuscular connective tissue. 

 The axial skeleton and surrounding connective tissue. 

 The muscular and connective-tissue wall of the alimentary 



tract. 

 Part of the peritoneal epithelium. 



A. B. 



(2) 

 (3) 

 (4) 



(2) 



(3) 

 (4) 



(5) 



Fig. 23. Sections thkodgh the trunk of a Sctllium embryo slightly 

 younger than 28 f. 



Figure A shews the separation of the body-cavity from the pericardial cavity by 

 a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the 

 narrow passage which remains connecting the two cavities. Fig. B through a posterior 

 part of the trunk shews the origin of the segmental tubes and of the primitive ova. 



sp.c. spinal canal; W. white matter of spinal cord; pr. commissure connecting 

 the posterior nerve-roots ; ch. notochord; x. sub-notochordal rod ; ao. aorta; si', sinus 

 venosus; can. cardinal vein; lit. heart; pp.hoAj-c&Yiiy; pc. pericardial cavity ; (zs. 

 sohd oesophagus; I. liver; mp. muscle-plate; vip'. inner layer of muscle-plate; Vr. rudi- 

 ment of vertebral body; st. segmental tube; sd. segmental duct; sp.v. spiral valve; 

 V. subintestinal vein. 



In the region of the head the mesoblast does not at first become 

 divided into somites ; but on the formation of the gill clefts a division 

 takes place, which is apparently equivalent to the segmentation 



