76 BIOLOGY OF THE LABORATORY MOUSE 



Observation of 608 heat periods in the rat showed an average duration 

 of 13.7 hours, with a range of i to 28 hours (12). Periods that start early 

 in the evening tend to run somewhat longer than ones that start later (31). 

 The duration has not been so accurately determined in mice, but is probably 

 much the same as in the rat. One estimate has placed it at about 12 hours 

 (126). 



In some cases what is commonly regarded as the estrous smear may last 

 for long periods. Allen (3) found that "as diagnosed by the smear method," 

 estrus usually lasts i or 2 days, but that unbroken estrous smears may con- 

 tinue for 9 days, and that 4 days of "heat" are not uncommon. These 

 cases of long continued estrous smear may be the result of the irritation due 

 to smearing (133), or they may occur normally in certain strains. The ce 

 strain commonly shows long intervals of cornification (Ossen, unpublished 

 data, 50). In any case they cannot be taken to indicate a long duration 

 of actual heat without further evidence. 



The modal length of the complete cycle is commonly 4 days in rats 

 (12), 5 days in mice. Parkes (loi) found the following distribution for 

 1000 cycles in unmated mice: 2 days, .4%; 3 days, 2.9%; 4 days, 15.8%; 

 5 days, 29.3%; 6 days, 21.8%; 7 days, 12.2%; 8 days, 6%; 9 days, 3.1%; 

 10 days or more up to 28 days, 8.5%. There seems to be a tendency for 

 the length of the cycle to increase with the age of the female (120). There 

 are marked strain differences in the length of the cycle (1,4, 24). Strain 

 differences are even more pronounced in the matter of the individual stages 

 of the cycle as indicated by the smear. The diestrus interval is commonly 

 the longest interval, and also the most variable. The daily changes in the 

 smear throughout three typical cycles are shown in Fig. 35. In these 

 particular cycles the approximate lengths of the different stages are: 

 proestrus, i day; estrus, }'2 day; metestrus-i, i day; metestrus-2, i day; 

 diestrus, 23^^ days. 



In the rat, low temperature has been shown to lengthen the cycle (16, 80). 



Ovulation. — Ovulation occurs spontaneously during estrus in both 

 mice and rats, whether mated or unmated. Different accounts differ 

 considerably as to the time of ovulation in relation to the onset of estrus, 

 a fact perhaps due in part to the existence of significant strain differences. 

 Ovulation in mice has been said to occur both at or near the beginning of 

 estrus (21, 81, 126), and at or near the end of estrus (3, 131). In a recent 

 study (126) with MacDowell-Bagg albino mice it was found that ovulation 

 usually occurred between 12 M. and 2 or 3 A.M., but in one case at least as 

 early as 11:30 P.M. and in another at least as late as 4:40 A.M. Since 



