60 



ANIMAL BIOLOGY 



arranged so that they form the surface of a hollow sphere of cells 

 known as a blastula. All the cells at first appear essentially 

 similar, but soon those at one side of the blastula become in- 



B 



D 



E 



F 



d 



GUI 



Fig. 31. — Early stages in the 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, ectoderm; 

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

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



vaginated until the central cavity, termed the blastocoel, is 

 largely obliterated. Accordingly there results the gastrula stage, 

 which may be roughly compared to a sac, composed of two layers 

 of cells with an opening to the exterior termed the blastopore. 

 The outer layer is known as the ectoderm, and the inner, which 



