56 



BIOLOGY OF THE LABORATORY MOUSE 



ijybrid stocks tend to have shorter gestation periods than inbred stocks. 

 The normal gestation period in the rat is shghtly longer than in the mouse, 

 ranging from 21 to 23 days (69). 



Table i 

 Length of Gestation Period in the C57 Black and dba Strains (Fekete, 



Unpublished Data) 



Birth of litters most commonly occurs at night (92). There is a decided 

 maximum in the number of births between midnight and 4 A.M., but 

 parturition. between 4 P.M. and midnight is not uncommon. Altogether, of 

 164 timedlDirths, two thirds occurred between 4 P.M. and 4 A.M. 



An estrus occurs about 20 hours after parturition, and while fertile 

 matings at this time are not common in some stocks of mice (35) unless the 

 newly arrived Htter is killed at birth, they occur quite regularly in other 

 stocks. Thus lactation and gestation may proceed simultaneously. Under 

 these circumstances the gestation period is lengthened, the extent of the 

 lengthening being slightly correlated with the number of suckHng young 

 With only one or two young suckling, the prolongation does not exceed 7 

 days, with three or more young suckling prolongations up to 12 or 13 days 

 are not uncommon. The maximum recorded is 16 days (20, 47, 51, 136). 

 Kirkham (73) has shown that the prolongation is due to a delay in implanta- 

 tion, which, instead of occurring during the fifth day post coitus as normally, 

 occurs on some later day, the embryos meantime lying free in the uterus. 

 Mating may occur during pregnancy (^s)^ but that such matings are accom- 

 panied by ovulation is open to doubt. 



Litter size. — ^Litter size differs greatly with the strain, with the age and 

 condition of the mother, and with order of the litter. Bittner (10) gives the 

 data reproduced in Table 2 which shows the relation between order of litter 

 and litter size for the highly inbred A strain. 



