264 



ANIMAL BIOLOGY 



The larger cells now sink into and nearly obliterate the central 

 cavity of the blastula, thus forming a typical gastrula stage com- 

 posed of two layers of cells, ectoderm on the outside and endoderm 

 on the inside. The infolded enteric pouch, or enteron, enclosing 



A 



B 



D 



E 



F 



d 



GUI 



Fig. 172. — General plan of the early development of the egg of an animal. 

 A-F, cleavage and formation of the blastula; G, section of blastula showing 

 the beginning of gastrulation; H-I, early and later gastrula stages, a, ecto- 

 derm; 6, endoderm; c, blastocoel; d, blastopore, leading into the enteric cavity; 

 e, cells, arising from the endoderm, destined to form the mesoderm. 



the enteric cavity, eventually becomes the main part of the ali- 

 mentary canal of the worm; its present opening to the exterior, 

 or blastopore, forming the mouth. So the developing worm has 

 now reached a transient state which is broadly comparable to 

 the permanent adult condition of Hydra. (Figs. 172, 173.) 



