58 



BIOLOGY OF THE LABORATORY MOUSE 



261 males and 262 females, an almost exact 1:1 ratio. MacDowell and 

 Lord (90) also present evidence that there is no continuous selective elimina- 

 tion of one sex or the other before birth. 



An alteration in the sex ratio through excessive breeding of the fathers 

 and through treatment of the fathers with alcohol has been both claimed and 

 denied (28, 57, 87, 104 and others). An effect through injection of the uterus 

 with sodium bicarbonate before breeding has also been claimed (14), and 

 there is evidence that diet may effect the ratio (11). 



Postnatal development. — Mice are born hairless, except for short 

 vibrissae, and with eyes and ears shut. Sex can be distinguished at birth: 

 males have the larger genital papilla, and there is a greater distance between 

 this and the anus in males than in females. At nine days females show five 

 pairs of conspicuous nipples, though these tend to be obscured in a few days 

 by the lengthening fur. The external ears have opened by three days. A 

 well developed coat is present at two weeks. At twelve to fourteen days a 

 number of interesting changes occur. There is a break in the growth curve, 

 the eyes open, the external ears commence a rapid growth, the first jnoult_ 

 begins, the larger follicles in the ovary develop an antrum, there is an 

 increase in muscular activity. At about the same time the young mice eat 

 their first solid food. 



Table 3 

 Data Indicating the Age at Which Maturity Is Reached by Fem.4les in 



Two Different Stocks 



In young mice the vagina is closed by a membrane. The age at opening 

 varies considerably both within and between stocks. In one series of 100 

 mice the age at opening ranged from 28 to 49 days with the median at 35 

 days (45). The first estrus as indicated by cornification of the vagina occurs 

 soon after vaginal introitus. In one set of observations the interval was 24 

 to 120 hours (96). However, estrus, in the sense of willingness to mate. 



