RECOGNITION OF SEX SEX RATIO BODY WEIGHT 



27 



As a rule the descent of the testes occurs about the fortieth day 

 of age or somewhat earlier. The following is a condensed form 

 of Jackson's table for the ano-genital distance. 



TABLE 8 

 Ano-genital distance in young albino rats of various ages 



8. Sex ratio. On the basis of 30 litters comprising 255 indi- 

 viduals, Cuenot ('99) reports among albino rats when the litters 

 are examined shortly after birth 105.6 males to each 100 females. 



King ('11 b)in 80 litters containing 452 individuals, found 107.3 

 males to 100 females, and in a later series of 120 litters (which 

 includes the 80 litters just mentioned) containing 690 individu- 

 als, a sex ratio of 107.8 males to 100 females. Finally, in a 

 group of 814 litters, comprising 5691 individuals, King and Stot- 

 senburg ('15) found 108.1 males to 100 females. 



In a thriving colony therefore a ratio of about 108 males it 

 to be expected. This however is subject to a seasonal variation. 

 At the two periods of greatest reproductive activity in the spring 

 (March-May) and again in the autumn (September-November) 

 the proportion of males (the sex ratio) is low. 



In the first litters of young females the sex ratio tends to be 

 higher than in the later litters but no relation of sex ratio to 

 size of litter has been found (King and Stotsenburg, '15). 



9. Body weight according to sex at maturity. At maturity 

 the body weight of the male Albino is much greater than that of 

 the female. According to our records for the common strain 

 ages not known the four largest males thus far examined 

 weighed 320, 327, 343 and 438 (fat) grams respectively, and the 

 four largest females 280, 287, 319 and 359 (fat) grams. In Albi- 



