94 



THE BIOLOGY OF THE FROG 



CHAP. 



smaller as the process of in-pushing is completed. The pro- 

 cess of gastrulation, which is exemplified in its typical form in 

 the development of a starfish or sea-urchin, becomes very 

 much modified in different animals. Such is the case in the 

 development of the frog. The large accumulation of yolk at 

 the vegetal side of the blastula prevents the invagination of 

 this region from taking place in the typical way. The same 



Fig. 19. — SagittaJ section through a frog embryo. B, blastocoel or segmen- 

 tation cavity; BP, lip of blastopore; EE, outer or epidermic layer of 

 ectoderm ; EN, inner or nervous layer of ectoderm ; Y, yolk cells. 

 (After Marshall.) 



end is reached partly by a process of in-pushing and partly 

 by the overgrowth of the white pole by the dark. The in- 

 pushing and overgrowth take place more on one side of the 

 egg than the other, and these processes are first indicated 

 by the appearance of a crescentic groove a little below the 

 equator of the egg. The crescent represents the beginning 

 of the blastopore. The groove is deepest at the center and 



