46 Veterinary Obstetrics 



The cells of the follicle multiplj' rapidly and, the growth of the 

 external layer being more rapid than the inner, there results a 

 separation between the two, except at the point of attachment of 

 the ovum, revealing upon section, a cresent-shaped cavity which is 

 filled with fluid. 



Fully developed, this constitutes the. Graafian follicle, which 

 consists-of the outer layer of follicular cells or tunica granulosa, 

 the inner mass of granular cells or discus proligerus and the ovum, 

 attached within or upon the inner cell mass. The cavity of the 

 follicle is occupied by the follicular fluid. The ripening egg sacs 

 lie at first deeply in the ovary ; as the size of the follicle increases 

 they approach more and more nearly to the surface and even pro- 

 ject beyond in varying degrees dependent somewhat upon species. 

 They vary greatly in size. In the mare they are not readily 

 observed upon the outer surface owing to the dense, heavy tunica 

 albuginea and to the fact that they ripen and rupture in the con- 

 cealed hilus instead of upon the free surface as in most other 

 species. The exposed surface of the ovary is frequently occupied 

 by massive cysts, or over-distended follicles. , 



In the cow, the ripe follicles are prominent upon the ovarian 

 surface while, in the sow, they project entirely beyond and main- 

 tain their connection by a constricted neck. As the follicle be- 

 comes more and more distended it ruptures at the weakest part 

 of its wall and discharges its ovum or ova, with the follicular 

 fluid and portions of its inner cellular layer, upon the surface of 

 the ovary, where the ovum is normally taken up at once by the 

 pavillion of the oviduct and conveyed toward the uterus. In 

 some cases, the ovum is not discharged promptly or even at all 

 from its follicle, but may become fecundated and developed there, 

 constituting ovarian pregnancy f which see), while in other in- 

 stances it possibly escapes free within the peritoneal cavity. 



During this period of maturation and rupture of the ovisac, the 

 ovum itself undergoes important changes and, prior to its dis- 

 charge from the follicle, its nucleus leaves its center and passes 

 toward the periphery ; a definite vitelline membrane is formed 

 within the zona radiata immediately about the egg : the nucleus 

 becomes indistinct and, while the yolk or vitellusjretracts slightly 

 from the vitelline membrane at one point, the first polar body, a 

 small mass apparently derived from an unequal division of the 

 nucleus, is assumed to be extruded, soon after which the 



