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 Utters comprising 255 indi- 

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

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



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

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

 includes the 80 Utters just mentioned) containing 690 individu- 

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

 group of 814 Utters, comprising 5691 mdividuals, 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 Utters of young females the sex ratio tends to be 

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

 size of Utter 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- 



