REPRODUCTIOX 75 



ing, numerous old corpora lutea are also present in females which have been 

 unmated for several previous cycles. 



The mammary glands. — The mammary glands show cyclic growth and 

 regression, though the changes are slight compared to those occurring during 

 pregnancy (30, 82). In proestrus buds appear on the ducts particularly 

 around the periphery of each gland, and large blunt projections appear on 

 the main ducts near the nipples. In estrus the mammary ducts become 

 dilated, and the buds formed during proestrus prolongate. Metestrus-i 

 introduces regressive changes and by the end of metestrus-2 the ducts are 

 decreased in width and the duct endings collapsed. In diestrus the mam- 

 mary gland consists of a very open network of narrow, thread-like ducts 

 with comparatively few branches, the branches themselves being simple. 



Other concomitants of estrus. — It has been noted that in the rat bodily 

 activity, as measured by the number of hourly revolutions of a rotating drum 

 placed in the cage, increases during estrus (65, 121, 135). A loss in weight 

 at estrus has been described in mice (i), but the weight cycle was not 

 regular except in mice with a very long estrous cycle (13-14 days), and it 

 does not seem to occur in rats (122). A cyclic change in the electrical 

 potential between the vagina and the symphysis pubis has been described 

 in rats (112). There is a marked peak in potential in late estrus, with an 

 abrupt fall when estrus terminates. A minor peak occurs about two days 

 before estrus. 



The postpartum estrus. — An estrus occurs in mice and rats within about 

 20 hours of parturition. The range for mice in the interval between 

 parturition and the following ovulation has been found to be about 14 to 

 28 hours (84). The cornification of the vagina is not complete at this 

 estrus, and the cornified cell content of the smear never reaches 100% 

 (Fig. 36). Fertile matings are less often obtained during this period than 

 during the course of the normal cycle. There is less fluid in the uterus than 

 during a normal estrus (93). 



The time relations of the cycle. — In the mouse the onset of heat usually 

 occurs in the night, most commonly between 10 P.M. and i A.M. Occa- 

 sionally it occurs between i and 7 A.M., in only rare instances during the 

 day (81, 126). Similar results have been obtained with the rat except that 

 the modal hour for the onset of estrus is several hours earlier, heat usually 

 beginning between 4 and 10 P.M. (12, 31, 121). The onset of heat may be 

 made to occur in the daytime in either mice or rats by keeping them in a 

 room that is dark in the daytime, lighted at night (23, 52, 65, 126). 



