RECOGNITION OF SEX IN THE YOUNG RAT. 173 



firmed the statement that the mammae (nipples) are readily 

 visible in the female of 14-15 days, and that after that time they 

 become hidden by the hair. During lactation, of course, they 

 again appear. I have also observed further that the nipples are 

 visible at an earlier age than that mentioned. During the entire 

 second week, and at least in some cases during the first week, they 

 can be noted by careful observation in the female. From the 

 time they first appear, the definite number (six pairs) is present. 



When the hair coat becomes well developed, at about 16 or 17 

 days of age (when also the eyes are opened), a fourth sexual 

 characteristic may be noted. In the male, a small area just 

 ventral to the anus remains bare. In later stages it becomes to 

 a certain extent covered with short hairs, but always remains 

 relatively bare. This corresponds to the dorsal part of the scrotal 

 area, as may be noted after the sixth week, when the testes 

 occasionally descend. In the female, there is a corresponding 

 bare, or relatively bare, strip which extends all the way from the 

 anus to the genital papilla (clitoris). The part of this bare strip 

 just dorsal to the clitoris corresponds to the vaginal aperture; 

 but the aperture does not appear until the middle or end of the 

 second month. In one case, a female bore a litter at the age of 

 10 weeks, and must therefore have become pregnant at about 

 7 weeks. Lantz 1 cites a case where a white rat is said to have 

 given birth to n young when only 8 weeks old, and which there- 

 fore must have bred when only 5 weeks old. 



It is evident from the foregoing that of the four characteristics 

 noted for distinguishing the sexes in young rats, one (visibility 

 of nipples) applies only before the age of about 16 days; one 

 (bare areas in ano-genital region) applies only after that age; 

 while the other two (ano-genital distance and size of genital 

 papilla) apply to all ages. 



1 David E. Lantz, Natural History of the Rat. In "The Rat and its Relation 

 to the Public Health," by various authors. P. H. and M. H. Service, Washington, 

 1910. 



