REPRODUCTION. 



175 



ject above, the surface of the ovary. The fluid contents of the 

 follicle increases and the wall becomes 

 thinner over it, until finally it bursts, 

 and the ovum with some of its sur- 

 rounding epithelium escapes. 



What peculiarity of the ovary favors 



the escape of the ovum? 

 The ovary is covered with a thin 

 layer of epithelium (the germinal epi- 

 theliurn)) and not by the serous mem- 

 brane which lines the abdominal sur- 

 faces of the rest of the viscera. This 

 is of great importance in the life of 

 the ovum, for it renders it possible for 

 it to enter the orifice of the Fallopian 

 tube without interfering with the peri- 

 toneum and without having to pass so 

 dense a structure. 



FIG. 53. 



Human Ovum, ruptured by 

 Pressure, showing the vitellus 

 partially expelled, the germi- 

 uative vesicle, with its germi- 

 uative spot, at a, and the 

 smooth fracture of the vitel- 

 line membrane. 



S 



Section of the Ovary (after Schrfin) : 1, outer covering; 1', attached border; 2, central 

 strorna ; 3, peripheral stroma ; 4, blood-vessels ; 5, Graafian follicles in their earliest 

 stage ; 6, 7, 8, more advanced follicles ; 9, an almost mature follicle ; 9', follicle from 

 which the ovum has escaped ; 10, corpus luteum. 



Whence is the ovum derived? 



It is a very highly developed cell, which is derived from the 

 germinal epithelium covering the ovary. In the development of 

 the ovary this epithelium dips into the surface of the organ, and a 



