^,NTE;XT-BOOK OF EMBRYOLOGY. 



measure to differences in the amounts of food or yolk stored within the egg. 

 Taking the human ovum as an example of ova containing a small amount of 

 yolk (deutoplasm) , it is not truly spherical in shape but ovoid, with an aver- 

 age diameter of slightly less than 0.2 mm. As seen in section in the ovary 

 it presents the appearance of the traditional typical cell (Fig. i). Surround- 

 ing the ovum is the zona pellucida, a thick, highly refractive membrane 

 which sometimes shows a faint radial striation. Immediately outside of this 



Zona 

 pellucida 



FIG. 



. From a section of the ovary of a 1 2-year old girl. The primary oocyte lies in a large 

 mature Graafian follicle and is surrounded by the cells of the " germ hill " (the inner edge of 

 which is shown in the upper left-hand corner of the figure). Photograph. 



membrane one or two layers of the epithelial cells of the Graafian follicle 

 are arranged radially as the corona radiata. The zona pellucida is probably 

 composed of differentiated cytoplasm of the inner ends of these cells. Some 

 investigators have described a delicate mtelline membrane between the zona 

 pellucida and the ovum ; others have not observed it. If this is present it is 

 probably a true cell membrane, a product of the egg cytoplasm. 



The egg cytoplasm (historically called the vitellus, whence the term 

 vitelline that is so frequently used in embryology) is more opaque and more 



