404 



GASTRULATION 



mediad and come to occupy the ventral lip of the blastopore, together with 

 the externally placed, epidermal cells (fig. 190G, arrow). The blastopore as 

 a whole grows smaller and moves to a dorsal position during the latter changes 

 (fig. 247 A-C). 



4) Closure of the Blastopore. See Chapter 10, neuralization in Amphioxus. 



Fig. 189. Gastrulation in Amphioxus. (Modified from Conklin, '32.) (A) Beginning 

 gastrula. (B) Observe that entodermal (hypoblast layer) is projected roughly in direc- 

 tion of future cephalic end of embryo. (C-G ) Observe continued projection of entoderm 

 toward cephalic end of future embryo. Note also position of polar body. In (F), (G), 

 and (H) the gastrula begins to elongate along the antero-posterior axis of the developing 

 embryo. (H) End of gastrular condition. Blastopore is closed by epidermal overgrowth, 

 and neurenteric canal is formed between archenteron and forming neural tube. 



