FIG. 11. SUCCESSIVE STAGES IN THE CLEAVAGE AND GASTRULA- 

 TION OF Amphioxus. A, one cell; B, two cells; C and D, four 

 cells; E, eight cells; F, sixteen cells; G, blastula stage of about 

 ninety-six cells; H, section through the same showing the 

 cleavage cavity; I, blastula seen from the left side showing 

 three zones of cells, viz., an upper clear zone of ectoderm, a 

 middle (faintly shaded) zone of mesoderm and a lower (deeply 

 shaded) zone of endoderm cells; J, section through the same 

 showing these three types of cells; K and L, successive stages 

 in the infolding of the endoderm; cells indicated as in the pre- 

 ceding figure. In all figures except D the polar body is shown 

 at the upper pole. Figs. A-H after Hatschek. a, anterior; 

 p, posterior; v, ventral; d, dorsal; be, blastocoel; gc, gastrocoel. 



