62 BIOLOGY OF THE LABORATORY MOUSE 



described as occurring occasionally in ovaries of both mouse and rat (43, 78). 



The earlier workers in this field were puzzled by the fact that while ova 

 formed before birth showed all the stages typical of meiotic prophase in the 

 male, these stages were not found in ova formed after birth. This problem 

 has been at least partly resolved by Swezy (130) in a study of the ovaries of 

 female rats from before birth to maturity. At five days post partum the 

 typical miotic prophase stages are, in fact, present. Deutobranch, lepto- 

 tene, synaptene, pachytene and diplotene nuclei can all be distinguished. 

 From then on the process is steadily modified and probably shortened. At 

 twenty days deutobranch nuclei and nuclei showing modified pachynema 

 stages may be seen. In the adult most of the different maturation phases 

 are lost altogether, or at least are not cytologically distinguishable. Crew 

 and Roller (32), however, have figured clear chiasmata in diplotene chromo- 

 somes in ova of mature female mice. This is excellent evidence that 

 synapsis (and crossing-over) has occurred, even though the stage at which it 

 occurs is difficult to see. Hence, however much the maturation stages may 

 be modified and telescoped in the developing ova of adult mice and rats, 

 there is little reason to doubt that they include the steps necessary for 

 accompHshing the pairing and crossing-over of the chromosomes required 

 by genetic evidence. 



While the concomitant occurrence of ovogenesis and ovular degeneration 

 at all ages until senihty is reached seems well-estabUshed, the rate of turn- 

 over, and the consequent length of fife of the individual ovum, remains some- 

 what uncertain. The view of early investigators that ova formed in the 

 embryo are functional in the adult has been largely abandoned, and some 

 writers have gone to the other extreme, maintaining that "individual 

 folUcles have a functional life span of only a day or two, in all cases less than 

 the length of the estrous cycle" (49). Lane and Davis (79), as a result of 

 studies of mitotic activity and volume changes in rat follicles, take an inter- 

 mediate position. They write as follows : " Folhcles less than 200 fx in diam- 

 eter are inactive mitotically and are thought to be physiologically quiescent." 

 In the adult, folhcles of this size or smaller "represent a reserve from which 

 are drawn succeeding crops of foUicles for maturation at succeeding estrous 

 periods. This follicle reserve will develop or be maintained without the 

 assistance of the pituitary, but for the production of follicles larger than 

 200 to 300 IX, pituitary assistance is required. . . . Between 200 and 300 /x 

 diameter, the follicle in any stage of the cycle shows mitotic activity in the 

 granulosa and theca which is shghtly augmented. These folhcles are 

 thought to be on the way to maturation or atretic degeneration. ... It 



