REPRODUCTION 



69 



The cyclical changes in the cell contents of the smear are shown in Figs. 

 35 and 36, taken from Voss (132). UnpubHshed data obtained at the 

 Jackson Laboratory indicate striking strain differences. However, the 

 cycles shown in Voss's figures may be taken as fairly typical. The smears 

 in terms of which the various stages of the cycle are defined are indicated 

 in Table 4. 



Of particular interest is the smear characteristic of estrus or heat. The 



complete or almost complete absence of leukocytes from the smear and 



the presence of cornified cells, in moderate numbers and not clumped, are the 



(usual criteria of estrus. In one study (126) with MacDowell-Bagg albino 



mice it was found that the smear at the onset of heat, as determined by 



PREG. 



LACTATION 



100% 



50%- 



0% 



DAYS 



10 



-I— I — I— r 



20» 

 C 



28 



A BCD 

 Fig. 36. — Graph showing the percentages of the three types of cells in the vaginal 

 smear of a mouse during a postpartum estrus, lactation, and the normal estrus follow- 

 ing lactation. Smears taken daily. cornified cells, nucleated epithelial 



cells leukocytes. (A) litter of four born and the beginning of lactation, (B) 



two young weaned, (C) one more young weaned, (D) last young weaned. Note the 

 incomplete cornification at the post partum estrus and the occurrence of a normal 

 estrus while one young is still nursing. {Voss 1930.) 



willingness to mate, still contained 5 to 75% of epithelial cells. The smear 

 marking the termination of estrus has not been so accurately determined, 

 but the presence of clumps or sheets of cornified cells is usually regarded as 

 marking the onset of metestrus. A typical late estrus or early metestrus 

 smear is shown in Fig. 34B. 



The vagina. — No part of the genital tract undergoes more striking 

 histological changes during the estrous cycle than the epithelium of the 

 vagina. The successive stages are shown in Figs. 37 and 38 and sum- 

 marized in Table 4. 



In proestrus the epithelium consists of three layers (Fig. 37A). The 

 outer layer is composed of epithelial cells sometimes more or less filled with 



