KECOGNITION OF SEX 



29 



development of hair on the ventral surface); (4) small extent 

 of the bare area just ventral to the anus (test applicable only 

 after the 16th day). 



The following is a condensed form of Jackson's table for the 

 anogenital distance. 



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

 day of age or somewhat earlier. 



9. Body weight according to sex. While the average weight 

 of the females at birth is less than that of the males, yet within 

 a few days after birth the females tend to grow more rapidly 

 than the males and may continue so to do during the following 

 thirty to forty days. Sometimes this leads to absolutely heavier 

 females as shown in table 157 and table 185 (King '23) for the 



TABLE 9 

 Ano-genital distance in young albino rats of various ages 



Norway rat. This relation appears to come about as a re- 

 sponse to less favorable conditions which tend to retard the 

 growth of the male more than that of the female. Under more 

 favorable conditions the female still grows somewhat faster than 

 the male during this period without however attaining or sur- 

 passing the body weight of the male. 



At 100 days of age the males weigh about 12 per cent more 

 than the females and this difference in body weight tends to 

 increase with advancing age, up to 18 per cent or more. The 

 sex difference is always found, but the amount of difference 

 varies according to the strain and other conditions. 



Maximum weights. In 1915 the maximum weights reported 

 were male 438 grams (fat); female 359 grams (fat). In The 



