46 Diseases of the Getiital Organs 



forms within the first, closely investing the ovum and con- 

 tinuous at one point with the cellular layer of the wall. 

 This mass of cells, bearing within it the permanent ovum, 

 constitutes the discus proligerus. 



The cells of the follicular walls multiply rapidly. The 

 external enveloping layer extends more rapidly than the 

 inner discus proligerus causing a separation between the 

 two, except at the point of attachment of the latter. A sec- 

 tion through the discus proligerus and follicle reveals a 

 crescent-shaped cavity filled with fluid. Fully developed, 

 this constitutes the ovisac, or Graafian follicle, which con- 

 sists of the outer layer of follicular cells, or tunica 

 granulosa, and the inner granular cells, the discus proli- 

 gerus, embedded within which lies the ovum. The cavity 

 of the follicle between the two masses of cells is occupied by 

 the follicular fluid. The immature egg sac usually lies 

 deep within the ovary. As it matures and the volume of 

 follicular fluid increases, it approaches the surface of the 

 ovary. In the cow about one-half of the ovisac protrudes 

 finally beyond the general ovarian surface as a hemisphere, 

 "hile the other half remains below the general level of the 

 gland. The ripe ovisac of the cow is about one-half to five- 

 eighths of an inch in diameter. In the sow the ovisac grows 

 completely beyond the ovarian surface and appears as a 

 pedunculated cyst one-fourth inch in diameter. In push- 

 ing toward and beyond the ovarian surface, the tunica al- 

 buginea, or ovarian capsule, atrophies and disappears at the 

 summit of the distended sac, and the peritoneum is pushed 

 outward. The follicular wall becomes very thin. During 

 rectal palpation in the cow, the ovisac often ruptures under 

 very slight pressure. During the maturation of the ovisac, 

 the ovum undergoes important changes. Before the ovisac 

 ruptures, the nucleus of the ovum passes from the center of 

 the egg toward its periphery ; a definite vitelline membrane 

 is formed within the zona radiata immediately about the 

 nucleus of the ovum; the nucleus becomes indistinct; and, 

 while the yolk retracts 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. 



