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Heredity and Environment 



FIG. 9. SUCCESSIVE STAGES IN 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 ; K 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 ; d, dorsal ; be, blas- 

 tocoel; gc, gastrocoel. 



