\\. 



72 BIOLOGY OF THE LABORATORY MOUSE 



exposed in some regions, not at all in others. During late proestrus and 

 throughout estrus cells are delaminated from the corniiied layer (Fig. 37B). 

 The onset of metestrus-i is characterized by the peeling off of the whole 

 layer, with an accompanying rise in the cornified cell count in the smear. 

 During metestrus-2 there is a rise in the nucleated cell count (Fig. 35), 

 indicating that in the last stages of the delamination process some of the 

 superficial layers of the stratum germinativum are included. The super- 

 ficial layers of the stratum germinativum, meantime, have become heavily 

 infiltrated with leukocytes (Fig. 38A) which also appear abundantly in the 

 smear at this time. As a result of the delamination of the superlicial 

 layers, the vaginal epithelium at diestrus contains only one layer, the 

 stratum germinativum, some three to seven cell layers in thickness (Fig. 

 38B). Late in diestrus active growth begins in the stratum germinativum, 

 and by proestrus a stratum granulosum has formed several cell layers below 

 the surface, thus completing the cycle. 



The uterus. — The uterus, like the vagina, undergoes a series of anabolic 

 and catabolic changes during the estrous cycle, but they are relatively 

 much less striking (Table 4 and Fig. 39). In proestrus and early estrus the 

 uterus shows marked hyperemia and is distended with fluid secreted by the 

 uterine glands. The distension starts to diminish in late estrus and in 

 diestrus the uterine wall is collapsed and anaemic. It has been reported 

 that in the rat the loss of some of this fluid is due to discharge into the 

 vagina (83). The uterine epithelium has been described as low columnar 

 in proestrus, with a distinct basement membrane, as high columnar in 

 estrus (3, 29). The increase in height is not marked, however, and in an 

 experiment with the rat a reverse change was noted accompanying the 

 increasing distension of the uterus (7). In metestrus-i degenerative proc- 

 esses become apparent. The basement membrane fades into a pink- 

 staining band which includes the basal sides of the epithelial cells and the 

 superficial stroma. The epithelium loses its definite organization and 

 shows vacuolar degeneration. Leukocytes appear in the region of the 

 basement membrane. In metestrum-2 the degeneration of the epithelium 

 is further advanced, so that almost all the epithelial cells are lost (no). 

 Cell walls at this stage are no longer recognizable and leukocytes are 

 numerous. The uterine glands show minimum activity. The onset of 

 diestrus is marked by the beginning of regenerative processes. 



The oviducts. — In the case of most mammals the oviducts show hypere- 

 mia at estrus (6, p. 668) and the same is probably true of mice, though the 

 condition seems not to have been specifically noted. There is none of the 



