ON THE RECOGNITION OF SEX THROUGH EXTERNAL 

 CHARACTERS IN THE YOUNG RAT. 



C. M. JACKSON, 

 UNIVERSITY OF MISSOURI. 



The rat is an animal widely used for scientific investigation. 

 Especially through the work on growth by Donaldson and his 

 associates, it has in some respects been more carefully studied 

 than any other animal. The recognition of sex from external 

 characters in the young living rats is therefore sometimes a 

 matter of considerable importance in selecting animals for various 

 lines of investigation. 



That considerable inconvenience has been occasioned by diffi- 

 culty in the early recognition of sex in the living rat is evident. 

 King, 1 for example, states (p. 385) that ' The sex of a newborn 

 rat cannot be ascertained with any degree of certainty unless the 

 animal is killed and dissected. When the rats are 14-16 days 

 old, however, the sexes are easily distinguished as Dr. Stotsenberg 

 has discovered, since the mammae are clearly visible at this time. 

 After this period the hair covers the entire body and it becomes 

 very difficult to distinguish the sexes in the living young until 

 they are several weeks old." Slonaker 2 similarly notes (p. 4) 

 that " Owing to the fact that it is difficult to determine the sex 

 of the young rats with accuracy the sexes were not distributed as 

 I would have wished." 



Since no method of distinguishing the sexes in the young rat 

 (except that noted by King) has, so far as I know, been published, 

 the following observations, which were made during a study of 

 growth in the white rat, may be of some interest and value. 

 While the observations were made exclusively upon the w r hite or 

 albino rat (Mus norvegicus albinus], they will doubtless apply 



1 Helen Dean King, "The Effects of Semi-Spaying and of Semi-Castration on 

 the Sex Ratio of the Albino Rat (Mus Norvegicus albinus)." Journal of Experi- 

 mental Zoology, Vol. 10, No. 4, 1911. 



2 James Rollin Slonaker, "The Effect of a Strictly Vegetable Diet on the Spon- 

 taneous Activity, the Rate of Growth, and the Longevity of the Albino Rat." 

 Leland Stanford Junior University Publications. University Series, April 2, 1912. 



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