28 BIOLOGY 



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). 



Cuenot ('99) reports a sex ratio of 105.6, while King and 

 Stotsenburg ('15) report 107.5 — without regard to the number 

 of the litter. This ratio is however subject to a seasonal varia- 

 tion. There is also a relation to the litter number as shown in 

 table 6 (King '24). 



The data in table 6 show that the sex ratio for one litter 

 group bears, seemingly, no relation to the ratio for the 

 group immediately preceding or following, and the differences 

 between successive ratios, when judged by their probable error, 

 are of little import. The ratios seem, however, to indicate that 

 the number of males tends to increase as the litter series ad- 

 vances up to the ninth litter. Beyond this point the relative 

 number of males decreases sharply, as the data for the last 

 four litters of the series, when combined, give a sex ratio of only 

 107.4 males to 100 females. , 



For the total of 4992 albino young recorded in table 6, the 

 sex ratio is 105.2 males to 100 females. This ratio is slightly 

 less than that given by Cuenot, and may, perhaps, serve as a 

 norm for sex ratio in the albino strain until a larger and more 

 complete series of data is available. 



It is to be noted that the sex ratio may vary according to 

 strain and that within the same strain it is also subject to wide 

 modification by selection (King, '18). 



Thus in the first twenty-five generations of inbred Albinos, 

 the first and second litters combined gave in series A a sex 

 ratio of 121.3, while in series B the sex ratio was 85.1. These 

 two series were from the same strain, but had been selected for 

 the high or low sex ratio respectively (King, '18). 



8. Recognition of sex. The recognition of sex through ex- 

 ternal characters in the young rat has been studied by Jackson 

 ('12). He finds the male may be recognized by (1) The larger 

 size of the genital papilla; (2) the greater anogenital distance 

 (see table 9); (3) the absence of clearly marked nipples. (This 

 test is applicable only up to the age of 16 days, i.e., before the 



