22 



Heredity and Environment 



* G 



Fig. 9. Successive Stages ln the Cleavage and Gastrulation of Am- 

 phioxus. 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; /, 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; /, section through the same showing these three 

 types of cells ; A." and L, successive stages in the infolding of the endo- 

 derm ; cells indicated as in the preceding figure. The polar body is shown 

 at the upper pole, a, anterior; p, posterior; v, ventral; (/, dorsal; be, blas- 

 toccel ; gc, gastroccel. 



