16 OUTLINES OF CHORDATE DEVELOPMENT 



H 



FIG. 6. Gastrulation in Amphioxus. After Cerfontaine. A. Blastula show- 

 ing 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 blastocoel in ventral lip of blastopore. G. Gastrulation completed. 

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

 established by overgrowth of neural folds. Continued mitosis in germ ring. 



