254 



Basic Structure of Vertebrates 



VEG 



Fig. 211. Gastrulation in the frog. (A, B, C) The whole embryo at successively 

 later stages; viewed toward the vegetal pole. (A', B', C'j Bepresent, in a somewhat 

 diagrammatic way, sections of corresponding stages cut in the plane including the 

 polar axis and bisecting the gastrular invagination, (I); this plane corresponds to 

 the median plane of the adult. During the latter part of the period of gastrulation, 

 as the 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. 215A). (a) Archenteron; (an) animal pole; (b) blastocoele; (bp) blastopore; 

 (ec) ectoderm, (en) endoderm; (i) invagination; (np) neural plate; (veg) 

 vegetal pole; (y) yolk; (yp) yolk plug. (Courtesy, Neal and Band: "Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



