184 EMBRYOLOGY. 



ting up of the vitellus or yelk, first in halves, then, in quarters, and 

 so on until the vitellus becomes a mass of minute granular-look- 

 ing nucleated cells (Fig. 56). The segmentation of the nucleus 

 continues with the corresponding change in the yelk. 



FIG. 56. 



Segmentation of the Vitellus in the Impregnated Egg of the Rabbit (Coste). 



Where does this change occur? 



In the Fallopian tube. The human ovum probably remains in 

 the Fallopian tube for eight or ten days before reaching the uterus, 

 and while there does not materially alter its appearance. Barring 

 the fact that there is added to its circumference an albuminous 

 layer, it is unchanged in external looks. 



What is the germinal membrane? 



The outer layers of vitelline cells after segmentation are closely 

 packed and resemble polygonal epithelial cells, and in fact form a 

 sort of lining to the zona pellucida, holding the liquid of the vitel- 

 lus in a central cavity. This lining is the germinal membrane or 

 blastoderm. 



Why does the segmentation of the vitellus take place ? 



This is not known, nor is it understood how it occurs, but it fol- 

 lows the union of the male and female elements: of Generation onlv. 



