176 



EMBRYOLOGY. 



certain portion is finally walled off by growth of the surface cells. 

 Thus a ball of epithelial cells is introduced into- the body of the 

 organ, and one cell develops the ovum, the rest going on to make 

 the Graafian follicle and the germ-hill. (See 5, 6, 7, 8, 9, 

 !g. 53.) 



Describe the ovum. 



It is a minute globular cell containing a nucleus and nucleolus. 

 In diameter it is a little less than y^ in. (Fig. 54). 



Semi-diagrammatic Representation of a Mammalian Ovum, highly magnified : zp, zona 

 pellucida ; vi, vitellus ; gv, germinal vesiale ; gs, germinal spot. 



What is the zona pellucida or vitelline membrane ? 



It is a thick hyaline membrane (cell-wall) which encloses the 

 cell. 



What is the vitellus ? 



It is a granular protoplasm which makes up the cell-body. 



What is the germinal vesicle ? 



It is the nucleus of the cell ; a somewhat large, transparent, and 

 well-defined body set somewhat eccentrically in the vitellus the 

 yolk of the egg. 



What is the germinative spot ? 



The nucleolus of the cell. In addition to the nearly transparent 



