12 OUTLINES OF CHORDATE DEVELOPMENT 



is slightly unequal and each of the four cells forms a smaller 

 cell toward the animal pole, and a larger toward the vegetal. 



FIG. 5. Cleavage in Amphioxus. After Cerfontaine. A. Four-cell stage 

 viewed from animal pole. The two antero-dorsal cells are the smaller. B. 

 Eight-cell stage viewed from animal pole showing the four sizes of the cells. C. 

 Sixteen cells viewed from the left side. D. Thirty-two cells viewed from 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, postero-ventral ; r, right; v, 

 vegetal pole. 



Thus in the eight-cell stage (Fig. 5, B) we have four pairs of 

 cells of four sizes; the four micromeres of the animal pole 





