172 



C. M. JACKSON. 



also to the ordinary gray or brown rat (Mus norvegicus). A few 

 observations upon young gray mice indicate that (at least with 

 respect to the ano-genital distance and the size of the genital 

 papilla) the same method may be utilized in distinguishing the 

 sexes in this and perhaps other members of the genus. 



TABLE I. 



ANO-GENITAL DISTANCE IN YOUNG ALBINO RATS OF VARIOUS AGES. 



The observations may be grouped under four headings. The 

 first and most important character to be noted is the ano-genital 

 distance, which is the distance measured from the anal aperture 

 to the base of the genital papilla (clitoris or penis). The meas- 

 urements are summarized in Table I. They were made on the 

 living animals at the various ages from birth to 6 or 7 weeks, 

 when the sexes are easily distinguished by the ordinary char- 

 acters. It will be observed that in all cases the ano-genital 

 distance in the male is much greater than in the female of the same 

 age. It averages nearly twice as great. Although, owing to 

 the great range of variation in the size of the body, the largest 

 distance recorded in the table for the female sometimes ap- 

 proaches the smallest found in the male for the same age, there 

 is never in any given litter the slightest difficulty by this measure- 

 ment in determining the sex with ease and accuracy. 



A second characteristic is found in the size of the genital papilla. 

 This (anlage of penis or clitoris) is always larger and more promi- 

 nent in the male than in the female. This difference is clearly 

 evident when the sexes are compared in individuals of the same 

 litter. 



A third characteristic is the presence of the mamma, already 

 mentioned by King as observed by Stotsenberg. I have con- 



