24 TEXT-BOOK OF EMBRYOLOGY. 



When the Graafian follicle is mature, having reached its maximum size, 

 it produces a bulge on the ovary; and there is only a thin membrane, com- 

 posed of the granular layer, the theca and the germinal epithelium of the 

 ovary, between the follicular cavity and the exterior of the ovary (Fig. 14). 

 At a certain time this membrane breaks and the follicular fluid gushes out, 

 carrying with it the ovum and some of the cells of the germ hill. The ovum 

 is then free in the abdominal cavity whence normally it passes into the 

 open end of the oviduct, or Fallopian tube. The cause of the rupture of the 

 follicle has not been ascertained; but there are certain facts which throw light 

 upon it. In the dog ovulation occurs during oestrus, or the period of "heat," 

 independently of approach of the male. In the mouse, the rat and the 



Germinal 



epithelium 



Germ hill Theca foUicu i 



with ovum (vascular layer) 



Theca folliculi (fibrous layer) 

 Stratum granulosum 



FIG. 14. From section of human ovary, showing mature Graafian follicle ready to rupture. 



Kollmann's Atlas. 



guinea-pig ovulation also occurs spontaneously during oestrus. In the 

 rabbit ovulation occurs about ten hours after coitus, and it has been shown 

 experimentally that the follicle does not rupture after any stimulus except 

 coitus. The sheep ovulates spontaneously during the earlier "heat "periods 

 of the breeding season, but in the later periods coitus seems necessary to 

 bring about the rupture of the follicle. In the bat, however, there are pecu- 

 liar circumstances: Copulation takes place in the autumn, the spermatozoa 

 remaining alive in the uterus until the following spring, and then ovulation 

 occurs apparently in response to seasonal temperature changes without even 

 a "heat" period. These are only a few instances out of a great number of 



