^82 Undischarged Ovarian Follicle 



gradually absorbed. Granules of black or brown pigment may be detected in 

 the thecal wall and these arc probably derived from the blood pigment in 

 process of absorption. 



Blood follicles are pnly very rarely found in other species of mammal and 

 are normal in none of them under any conditions. Blood spots, however, are 

 found in the ovaries of the immature mouse as a result of injecting gonado- 

 trophic hormone (in pregnancy urine); these spots occur in the ovarian stroma 

 and, though some may be large and easily seen, others are very minute (Rob- 

 son*); moreover, it is not always clear that they are associated directly with 

 follicles. 



It used to be supposed that hemorrhagic follicles in the rabbit represented 

 follicles which failed to undergo ovulation owing to the buck being withheld 

 and that such was probably the normal fate of mature follicles after an estrous 

 period had been going on for a prolonged time. This conclusion, however, 

 is now known to be incorrect, in view particularly of two considerations. First, 

 it is now realized that the formation of blood follicles is the result of a sexual 

 stimulus of the nature of an orgasm and may be produced artificially by the 

 injection of anterior pituitary secretion or pituitarylike substances or by 

 electrical or other stimuli which act on the pituitary through the intermedi- 

 ation of the central nervous system. Secondly, it has been shown through the 

 work of Evans and Swezy° that in the rat, guinea pig, dog, cat, and mouse 

 oogenesis occurs rhythmically throughout the whole period of sexual life and 

 in relation to the ovulation cycle. It had already been ascertained, contrary 

 to what was usually supposed, that in the rabbit (Lane-Claypon") and other 

 animals new ova were produced after sexual maturity, and the earlier litera- 

 ture is reviewed by Evans and Swezy. Moreover, Hill and White, ^ and Smelser, 

 Walton and Whetham® independently discovered that the mature follicle does 

 not persist in the estrous rabbit but is rather short-lived, so that during the 

 prolonged heat periods numbers of follicles appear and disappear, the series 

 overlapping in such a way that at any one time there are approximately the 

 same nimiber of mature follicles on the ovarian surface and estrus and po- 

 tential fertility are continuous. From what has been said it is clear that the 

 formation of hemorrhagic follicles in the rabbit is of the nature of an at- 

 tempted ovulation and probably occurs normally as a result of a sexual stimu- 

 lus of some kind (for instance, when two female rabbits are kept together) 

 which is, however, insufficient to produce ovulation. 



In other species of mammals which are known to ovulate only after coition, 

 hemorrhagic follicles are not formed, or at any rate have not been described 

 except rarely or as occurring abnormally. In the American ground squirrel 

 Foster^ has given an account of follicular atrophy of the more usual type in 

 which there is no hemorrhage, and such atrophy appears to be the fate of the 

 mature follicles which do not discharge. The ovum, however, is stated to de- 

 generate before the surrounding epithelial cells, whereas in most mammals 

 the reverse is usually the case. 



