EARLY DEVELOPMENT OF AMPHIOXUS. 



37 



ever, during the fourth cleavage, some of the cells dividing before others, with 

 the result that numbers other than those just given will be found. The 

 smallest cells, with the least amount of yolk are the first to divide and they 

 divide more rapidly than the large cells with a greater yolk content; the inert 

 non-protoplasmic substance retards the progress of division. 



FIG. 20. Cleavage in Amphioxus. Cerfontaine, from Kellicott. 



A, four-cell stage seen from animal pole; B, eight-cell stage seen from animal pole, showing four 

 sizes of blastomeres; C, sixteen-cell stage seen from left side; A, thirty-two-cell stage seen 

 from vegetal pole; E, 32-64 cells seen from antero-dorsal region; F, half of early blastula 

 containing about 128 cells, a, Animal pole; ad, antero-dorsal; I, left; pv, postero-ventral; 

 r, right; v, vegetal pole. 



Division succeeds division in the blastomeres, with the irregularity 

 noted in the preceding paragraph. The cleavage planes vary considerably in 

 direction in different individuals. At the i6-cell stage the micromere group 

 assumes a sort of dome form and the macromere group in similar form fits 

 into the hollow of the dome (Fig. 20, C) . The early blastomeres remain well 



