191 1-] LOEB— CYCLIC CHANGES IN THE OVARY. 229 



by a spermatozoon form an embr}o in the uterine wall. Xew fol- 

 licles situated in the periphery of the ovary grow constantly to a cer- 

 tain size and then degeneration sets in. The lining granulosa cells 

 disintegrate and connective tissue grows into the cavity of the follicle. 

 The ova in these degenerating follicles undergo frequently matura- 

 tion and a few more or less regular divisions and then die. While 

 thus the majority of follicles degenerate, become atretic, before they 

 have reached maturity, a few follicles undergo certain progressive 

 changes and become mature. They may rupture and discharge the 

 ovum ; such a rupture is called an ovulation. The process connected 

 with ovulation causes a degeneration of all with exception of the 

 smallest follicles. These follicles grow and in the course of the next 

 six days they have reached that size at which degeneration may set 

 in. We find, therefore, degenerating follicles from the seventh day 

 after ovulation up to the time of the following ovulation. While 

 after the seventh day medium-sized follicles constantly degenerate, 

 new follicles grow and take the place of the degenerating disappear- 

 ing ones. It seems that it takes approximately ten days until some 

 of the follicles reach their full size. 



A\'e may therefore distinguish two periods in the ovarian cycle: 

 First, the period of growth extending over the first seven days fol- 

 lowing ovulation, and, secondly, the period of equilibrium in which 

 new follicles take the place of degenerating ones. After the first 

 large follicles have appeared, it takes a few days longer until large 

 follicles become transformed into mature follicles that are ready to 

 rupture. We find, therefore, the first mature follicles to appear 

 approximately eleven to thirteen days after ovulation and it would 

 be natural to expect that about fourteen days after the preceding a 

 new ovulation should take place. 



The sexual period — that is the period between two ovulations — 

 should therefore have a natural duration of approximately two weeks 

 in the guinea-pig. This is, however, not the case. The sexual 

 period in this species actually lasts about twenty to twenty-five days. 

 And this is due to the fact that a mechanism exists within the ovary 

 that prolongs the sexual cycle. In order to understand this mech- 

 anism, we must follow the fate of the ruptured follicle. A follicle 



