338 



LIFE HISTORY 



These observations however do not indicate the sex ratio at 

 birth. 



For the total of 1862 Norway young recorded' by King ('24) 

 in table 184 the sex ratio is but 85.8 males to 100 females. This 

 ratio is in accord with the only other sex ratio for the Norway 

 strain that has as yet been determined, namely, the ratio of 

 82.1 males to 100 females found by Miller in five litters comprising 

 fifty-one young. The sex ratio for the Norway strain is 19.4 ±3.34 

 points lower than that found for the stock albino strain (table 6) , 

 thus indicating that there is a significant difference between the 

 two strains as regards the relative proportions of the sexes in the 

 young at birth. 



TABLE 184 



Data for entire series of litters cast by 88 Norway females, born and reared in captivity 



It may be objected that the sex ratio found in this series of 

 Norways is due to the effects of captivity on the breeding females. 



Data obtained in later generations of these Norways make 

 this explanation seem improbable, and indicate that the low sex 

 ratio in the Norways is probably normal for the strain. 



Opening of eyes. Miller ('11) found the eyes to open at 16 

 or 17 days and also states that the young are weaned during the 

 sixth week. Observations at The Institute are in agreement with 

 these statements. 



Suckling. The lactation period tends to be longer in the 

 Norway than in the Albino. The range is wide but about five 

 weeks seems to be the normal suckling period in the caged 

 Norway. (King MS). 



