Fig. 17. — Gastrulation of Amphioxus. A. Blastula showing flattening of the 

 vegetal pole and rapid proliferation of cells in the posterior region (germ ring). 

 B. Flattening more pronounced; mitosis in cells of germ ring. C. Commencement 

 of the infolding (invagination) of the cells of the vegetal pole. C. Continued in- 

 folding, and inflection, or involution, of ectoderm cells in the dorsal lip of the 

 blastopore. The blastocoel becoming obUterated and the archenteron being estab- 

 lished. E. Invagination complete. Continued involution of the dorsal lip of 

 blastopore. Mitoses in germ ring. F. Constriction of blastopore and commence- 

 ment of elongation of the gastrula. Renmants of blastocool in ventral lip of 

 blastopore. H. Neurenteric canal established by overgrowth of neural folds. 

 Continued mitosis in germ ring, a, animal pole; ar, archenteron; 6, blastoporal 

 opening; ch, rudiment of notochord; dl, dorsal lip of blastopore; ec, ectoderm; 

 en, endoderm; gr, germ ring; nc, neurenteric canal; nf, neural fold; np, neural 

 plate; s, blastocoel or segmentation cavity; v, vegetal pole; vl, ventral lip of 

 blastopore; II, second polar body. (From Kellicott's " Outlines of Chordate 

 Development " [Henry Holt and Company] after Cerfontaine.) 



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