THE PHYLUM CHORDATA 



43 



animal pole and larger endodermal cells at the vegetal V)le. The 

 endoderm cells soon cave into the dome of ectoderm cells and/orm 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 3$. 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; I, left; pv, posterior ventral; r, right; v, vegetal pole. (From 

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

 after Cerfontaine.) 



