EARLY DEVELOPMENT OF THE FROG. 



57 



FIG. 31. Median sagittal sections showing successive stages of gastrulation in the frog's egg. 



Bracket, from Kellicott. 



A, beginning of gastrulation; B, slight advance in invagination and beginning of epiboly; C, 

 invagination and epiboly progressing, inflection of cells (involution) occurring around dorsal 

 lip of blastopore which is now an obvious structure; D, epiboly has resulted in covering of a 

 large part of yolk by lip of blastopore; E, blastopore is now circular and filled with the yolk 

 plug (cf. Fig. 30, A, 4) and the archenteron appears as a small space; F, the blastoccel is 

 nearly obliterated; G, gastrulation completed. 



a, Archenteron; b, blastopore; c, rudiment of notocord; ec, ectoderm; en, entoderm; gc, gastrular 

 cleavage, ge, entoderm (protentoderm) ; m, peristomal mesoderm; np, neural plate; w/, 

 transverse neural ridge; s, blastocoel. 



