BLASTULA, GASTRULA, AND GERM LAYERS 335 



FIG. 151. Gastrulation in Amphioxus. After Cerfontaine. A. Blastula 

 showing flattening of the vegetal pole and the rapid proliferation of cells in the 

 postero-dorsal region (germ ring). B. Flattening more pronounced; mitoses 

 in cells of germ ring. C. Commencement of the infolding (invagination) of the 

 cells of the vegetal pole. D. Continued infolding and inflection, or involution, 

 of ectoderm cells in the dorsal lip of the blastopore. The blastoccel becoming 

 obliterated and the archenteron being established. E. Invagination complete. 

 Continued involution in the dorsal lip of blastopore. Mitoses in germ ring. 

 F. Constriction of blastopore and commencement of elongation of the gastrula. 

 Remnants of blastoccel in ventral lip of blastopore. G. Gastrulation completed. 

 Continued elongation, and narrowing of blastopore. H. Neurenteric canal 



