THE PHYLUM CHORDATA 



43 



animal pole and larger endodermal cells at the vegetal pole. The 

 endoderm cells soon cave into the dome of ectoderm cells and form an 

 incipient gastrula as in Fig. 17, A, B, C. This invagination continues 

 until a typical embolic gastrula is formed as in Figs. 17, D and E. A 



Fig. 16. — Cleavage of Amphioxus. A. Four-cell stage viewed from the animal 

 pole. The two antero-dorsal cells are the smaller. B. Eight-cell stage viewed 

 from the animal pole showing the four sizes of cells. C. Sixteen-cells viewed 

 from the left side. D. Thirty-two cells viewed from the vegetal pole. E. Thirty- 

 two passing into sixty-four cells, viewed from the antero-dorsal region. F. Optical 

 section of right half of young blastula. About 128 cells, a, animal pole; ad, 

 antero-dorsal; /, left; pv, posterior ventral; r, right; v, vegetal pole. (From 

 Kellicott's "Outlines of Chordate Development" [Henry Holt and Company] 

 after Cerfontaine.) 



