REPRODUCTION 



67 



the superficial germ-ring region; the deeper wall of the invagination 

 consists of yolk cells. The groove, initiated as a slight invagination, 



AN 



VEG 



Fig. 44. — Gastrulation in the Frog. A, B, C, the whole embryo at successively later 

 stages; viewed toward the vegetal pole. A', B' and C represent, in somewhat diagram- 

 matic way, sections of corresponding stages cut in the plane including the polar axis 

 and bisecting the gastrular invagination, /; this plane corresponds to the median plane 

 of the adult. During the latter part of the period of gastrulation, as result of shifting 

 of the heavy yolk (compare B' and C), the embryo rotates so that the axis passing 

 through the blastopore (BP) becomes horizontal (see Fig. 48A). A, archenteron; 

 AN, animal pole; B, blastocoele; BP, blastopore; EC, ectoderm, EN, endoderm; /, 

 invagination; NP, neural plate; VEG, vegetal pole; Y, yolk; YP, yolk plug. 



now rapidly deepens (Fig. 4.4B, B'), not by continued invagination, but by 

 active growth of that upper (for later events prove it to be dorsal) lip 



