46 



CHORDATE ANATOMY 



VEG 



Fig. 40. — Gastrulation in the frog, 

 stages; viewed toward the vegetal pole. 



A, B, C, the whole embryo at successively later 

 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. 44-4). A, arohenteron; 

 AN, animal pole; B, blastocoele; BP, blastopore; EC, ectoderm, EN, endoderm; 1, 

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



