X.] 



BLASTODERMIC VESICLE. 



315 



about O09 mm. the size of the ovum at the close 

 segmentation to about 0*28 in diameter. It is en- 

 closed by the zona radiata and the albuminous layer 



FIG. 



BABBIT'S OVUM BETWEEN 70 90 HOURS AFTER IMPREGNATION. 

 (After E. van Beneden.) 



bv. cavity of blastodermic vesicle (yolk-sac) ; ep. outer layer ; 

 hy. inner mass ; Zp. albuminous envelope. 



around it. The blastodermic vesicle continues to 

 enlarge rapidly, and during the process the inner mass 

 undergoes important changes. It spreads out on the 

 inner side of the outer layer and at the same time loses 

 its lens-like form and becomes flattened. The central 



