INTERNAL ORGANS OF GENERA TION. 



51 



according to the species of animal, show evidence of increased vascularity, 

 and become distended ; the ovisac thins at the most prominent part to 

 which the ovum tends, and blood is extravasated into it ; then, partly by 

 absorption and partly by pressure, the coverings give way, the proligerous 



Fig. 25. 

 Graafian Vesicle and Ovum- 



disc and ovum escape outwards, and are either received into the Fallopian 

 tube for conveyance to the uterus, or, which is rare, fall into the cavity of 

 the abdomen. 



The size of the mature ovum in the Cow is ^io* ^^Z 2 o? Bitch t^^j ^^.t 

 ^\-Q, Rabbit -y\-q of an inch. 



Fig. 26. 

 Escape of Ovum from Ovisac 



After the rupture of a Graafian vesicle and the escape of the ovum, 

 the cavity of the ovisac is filled with a clot of blood, while its walls are 

 thickened and altered in color, being in most animals partially everted at 

 the ruptured orifice. In the Cow and Sheep the follicle has a brick-red 

 color, and in the Pig it is yellowish-brown — it is then designated in the 

 human subject, from its color, the corpus luteum ; but gradually the clot 

 shrinks, loses its tint, and the cavity contracts ; at the same time the 

 walls are hypertrophied, and the tunica granulosa becomes wrinkled and 

 transformed into cylindrical epithelium. By the time the succeeding 

 ovisac with the ripening ovum has begun to protrude from the surface of 

 the ovary, the old ovisac has lost its color with much of its dimensions, 

 and fallen inwards ; the cylindrical epithelium are infiltrated with fat and 

 are gradually absorbed. This change, with collapse of the wall, depresses 

 the cicatrix of the aperture ; and the successive shrinkings and cicatriza- 



