8 COMPARATIVE ANATOMY 



somatopleure, and the latter, together with the hypoblast, the 

 splanchnopleure. The cavity separating these is the body cavity, 

 or coelome (Fig. 7), 1 and is lined by an epithelium. The dorsal 

 part of the mesoblast which lies on either side of the middle line 

 early becomes transversely segmented to form a series of mesoblastic 

 somites or protovertebrce, which lose their cavities (Fig. 6, A and B) 

 and are concerned in the formation of the vertebral column, body 

 muscles, and urinogenital apparatus. 



As a general rule a thickened disc-shaped region can be recog- 

 nised at a certain stage of development on the dorsal pole of the 



KW 



FIG. 7. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE BODY or AN ADULT 



VERTEBRATE. 



Med, spinal cord ; JV7?, neural tube ; K W, body-wall ; Co, dermis ; Ep, endodermic 

 epithelium of alimentary canal (intestine) ; VR, visceral tube ; Ao, aorta ; 

 Ms, mesentery ; Per, parietal layer of the peritoneum ; Per 1 , visceral layer of 

 the peritoneum ; Mac, muscular coat of intestine ; Siibm, connective-tissue coat 

 of intestine ; DH, lumen of intestine ; W, vertebral centrum with dorsal arch. 



oosperm : this is the so-called embryonic area, and on it the first 

 indications of the body are seen. This region gradually becomes 

 constricted off from the yolk by the formation of furrows at its 

 anterior and posterior ends as well as laterally, and consequently 

 the connection of the body-rudiment with the ventral yolk-sac (the 



1 The coelome may arise as a segmentally arranged series of pouches 

 (enterocceles) from the archenteron, in which case its lining epithelium is at first 

 continuous with the hypoblast, as is most plainly seen in Amphioxus ; or it may 

 be formed secondarily by a splitting (delamination) of the mesoblastic tissue 

 (schizoccde). The first of these must be considered as the more primitive. 



