REPRODUCTION 63 



seems significant that the follicles in the size range of 401 to 500 /i should 

 exhibit the maximum activity in the granulosa. It is possible that these 

 constitute the group which will ovulate at the succeeding estrous period. 

 Numerically there are 6 of these follicles in the average metestrus ovary. 

 Allowing for atresia this number could easily produce the 3 to 5 ovulating 

 follicles which are present in each ovary at estrous." 



A rough calculation of the length of life of the average follicle is possible 

 from pubhshed data. In two experiments (98, 125) female mice were 

 irradiated with x-rays and the condition of the ovaries determined by breed- 

 ing tests or by histological study. These experiments show that fertile 

 matings may be obtained from females irradiated with a dose of 260 r for a 

 period not exceeding 28 days following treatment. After this they become 

 completely sterile, presumably because no new ova are prohferated by the 

 germinal epithelium (125). That irradiation causes early cessation of 

 ovogenesis is shown by the fact that small or primary follicles are absent in 

 ovaries of irradiated mice (150 r) killed 2 days after treatment. At 21 days 

 only a few normal follicles of the older types are present, the gland being 

 mainly composed of degenerating remnants. At 43 days there is a total 

 absence of all follicular structure (98). These two experiments show that, at 

 least in irradiated ovaries, ova can survive for only about 28 days following 

 their proliferation by the germinal epithelium. 



Further evidence as to the rate of development of ova is provided by 

 experiments in which rat ovaries were ligated, so that degeneration resulted 

 from loss of blood supply, followed by regeneration when the circulation was 

 re-established (26). At 8 or 10 days degeneration is usually complete 

 except for small peripheral regions. At 12 days small ova may be seen 

 recently differentiated from the germinal epithelium. At 21 days some 

 medium sized foUicles are present. At 30-34 days recovery is practically 

 complete, and the ovary contains fully developed Graafian follicles. The 

 interval from the first appearance of new ova to their final full development 

 is thus 18 to 22 days. This may be taken as the minimum time required for 

 the complete development of ovum and follicle. It is, of course, possible 

 that ova and follicles in normal ovaries develop at different rates and survive 

 for shorter or longer periods than do ova and follicles in ovaries under the 

 experimental conditions described above. The available evidence, however, 

 points to 18 to 28 days as a reasonable estimate of the time taken for the 

 mouse or rat ovum to mature following its separation from the germinal 

 epithelium. A quite difTerent line of evidence is provided by experiments 

 with ovaries of embryonic or new born rats and mice grown in vitro. Under 



