CEPHALOCHOEDA 147 



the coelomic segments. The coelomic segments arise as 

 paired expansions of the endoderm on each side of the noto- 

 chord. The first pair originate with the primitive front end 

 of the notochord. The successive pairs of pouches remain 

 for a short time in communication with the enteron, but 

 successively they are separated by a fusion which restores the 

 enteron after these structures have been developed. 



Notochord Neural plate 



Notochord Coelomic segment 



FIG. 71. Amphioxus. Transverse sections to illustrate the formation of the 

 nerve cord, notochord, and coelomio segments. After Hatschek. 



The walls of the somites are applied, the internal to the 

 nerve cord, notochord and endoderm, and the outer to the 

 ectoderm. The pouches extend downwards until they meet 

 below the endoderm, where they mutually form a mesentery. 

 This mesentery is absorbed, and a constriction separates the 

 pouches into a dorsal and a ventral cavity. The upper in 

 each is the myocoel and retains its meristic segmentation ; 

 they form the series of somites or myotomes. The lower is 

 the splanchnocoel, and becomes a continuous cavity by the 

 absorption of the walls which separate them. The coelom is 

 thus separated into the dorsal segmental myocoels and the 

 ventral splanchnocoel, which is the body cavity as usually 



