74 



BIOLOGY OF THE LABORATORY MOUSE 



Parks (21) have made a study of the rate of growth of maturing folUcles in 

 unmated mice. They find that the folHcles which will ovulate at the 

 following estrus are, on the average, only 380^1 in diameter at the beginning 

 of the estrous cycle. The follicles reach a maximum size of, on an average, 

 550 ^i in diameter immediately before rupturing. Most of this growth, 

 according to their study, occurs in the last 48 hours, during which period 

 the follicles increase 45% in diameter. Secretion of secondary liquor 

 folliculi, which at this time begins to replace the less fluid primary liquor 

 folliculi, may play a considerable role in the increase in size (6, p. 458; 131). 

 Just prior to ovulation the follicles bulge conspicuously from the surface of 

 the ovary. 



Table 5 



Average Distribution of Follicles According to Size throughout the 



EsTROus Cycle in the Rat (From Lane and Davis) 



* The percentage of the total follicle content which falls in a given size range. 



Several studies (4, 5, 79) have shown that the mitotic activity in the 

 ovary is cyclic, reaching a peak at estrus or metestrus- 1 (Fig. 39). Since 

 mitosis in the vaginal epithelium reaches a peak in proestrus or early estrus 

 (3, 83), it appears that the ovary responds to the estrus stimulus more 

 slowly than the vagina. Follicular atresia, like follicular growth, is cyclic, 

 reaching a peak in metestrus-2 (Fig. 39). 



In metestrus- 1 newly formed corpora lutea are present. Since corpora 

 lutea in unmated mice persist for. two, three or four cycles before disappear- 



