EL A 8 MO BRA NCHII. 



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. 



(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 and surrounding connective tissue. 

 (1) The muscular and connective-tissue wall of the alimentary 



tract. 



(5) Part of the peritoneal epithelium. 



A. B. 



sp.c 



ch~ 



ht 



- -Sp.f. 



FIG. 23. SECTIONS THROUGH THE TRUNK OP A SCYLLIUM EMBRYO SLIGHTLY 



YOUNOER 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 ; cli. notochord; x. sub-notochordal rod; ao. aorta; sv. sinus 

 venosus; car. cardinal vein; lit. heart; pp. body-cavity; pc. pericardial cavity ; as. 

 solid oesophagus ; /.liver; mp. muscle-plate; mp'. inner layer of muscle-plate; Vr. rudi- 

 ment of vertebral body; st. segmental tube; siL segmental duct; s/>. c. spiral valve; 

 r. subiutestinal 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 



