MAMMALIAN REPRODUCTIVE CYCLE 



505 



4 8 12 



DAYS AFTER BEGINNING OF ESTRUS 



Fig. 8.4. A schematic repiesentation of the folhcuhir cycle in the guinea pig. The heavj^ 

 sohd curve represents the diameters of the largest follicles, recalculated from the data of 

 Myers, Young and Dempsey (1936). The arrow point indicates ovulation. The other solid 

 curves and broken lines represent impressionistically the growth and atresia, respectivelj^ 

 of other groups of follicles that are not ordinarily destined for ovulation. 



11th or 12th day the largest follicles (ca. 

 800 /x) are "competent," i.e., capable of 

 being ovulated (Dempsey, Hertz and 

 Young, 1936; Dempsey, 1937). While the 

 largest follicles are developing to this stage, 

 multitudes of others begin to grow, being 

 carried on to various stages of development 

 before regression sets in. 



This pattern of the follicular cycle seems 

 to be generally true among mammals that 

 have been carefully studied, when allowance 

 is made for the fact that from one species to 

 another the characteristic maxima of follicle 

 diameter are extremely variable (shrew, 350 

 fjL-, rat, 900 /*; cow, 19,000 /x; mare 70,000 /x; 

 Asdell, 1946). In ovulatory cycles of poly- 

 estrous animals the greater part of follicle 

 growth is accomplished while the luteal 

 phase of the preceding cycle is in progress. 

 In successive anovulatory cycles like those 

 of the cat the patterns of the follicular cy- 

 cles are probably much the same (Evans 

 and Swezy, 1931 » . In the rabbit and ferret, 

 where more or less constant estrus char- 



acterizes the isolated females in season, 

 there is probably considerable telescoping 

 of successive waves of follicle growth such 

 that as one set of follicles begins to undergo 

 atresia another set is ready to take its place 

 (Hill and White, 1933). The difference be- 

 tween cat cycles and rabbit cycles seems to 

 be chiefly one of degree. The writer has seen 

 both types represented in persistent-estrous 

 rats, among litter mates of inbred strains 

 (Everett, 1939, and unpublished). 



At the end of the luteal phase of the 

 cycle in polyestrous animals there are al- 

 ready present several competent follicles 

 among an extensive population of smaller 

 ones. For example, the guinea pig corpus 

 luteum usually shows signs of regression on 

 day 13 of the cycle. It has been proved that 

 ovulation can be induced as early as day 

 12 by injection of LH (Dempsey, 1937), 

 several days earlier than it would normalh^ 

 occur (Fig. 8.5). In the human and monkey 

 it is possible that the "preferred" follicles 

 are recognizable by their larger size during 



