SEX RATIO OF THE ALBINO RAT 385 



kinds of germ cells in the gonads, this difference should be shown 

 by a distinct alteration of the normal sex ratio among the young 

 produced by mating animals from which one of the gonads had 

 been removed. To test this point a series of experiments on 

 the albino rat (Mus norvegicus, albinus) was started in the fall 

 of 1909. The results obtained in these experiments are given in 

 the present paper. 



The rats used in this series of experiments were operated upon 

 by Dr. J. M. Stotsenberg of The Wistar Institute, to whom I 

 am greatly indebted for this assistance. In all cases the operation 

 was performed on the rats when they were 16-20 days old, the 

 ovary or the testicle being removed while the animal was under 

 the influence of ether. Full details regarding the manner in 

 which the operations were made will be given in a forthcoming 

 paper by Dr. Stotsenberg. About half an hour after the opera- 

 tion the young rats were returned to their nest, and they remained 

 with their mother until they were one month old when they were 

 fully able to care for themselves. The sexes were separated when 

 the animals were two months old; and the rats w^re mated for 

 the first time when they were about four months old. Each pair 

 of breeding animals, earmarked for identification, occupied one 

 of the standard cages used for the rat colony of The Wistar 

 Institute. It was not possible, therefore, for the experiments to 

 be invalidated by promiscuous breeding. 



The sex of a newborn rat cannot be ascertained with any degree 

 of certainty unless the animal is killed and dissected. When 

 rats are 14-16 days old, however, the sexes are easily distinguished, 

 as Dr. Stotsenberg has discovered, since the mammae in the 

 females are clearly visible at this time. After this period the 

 hair covers the entire body, and it becomes very difficult to dis- 

 tinguish the sexes in the living young until they are several weeks 

 old. 



Cu^not ('99) ascertained the sex of 255 young albino ratB 

 belonging to 30 different litters. He found a slightly greater 

 number of males than of females ; the sex ratio being 105 . 64 males 

 to 100 females. Records that I have made of the sex of 452 

 young albino rats, belonging to 80 litters, give a sex ratio of 107 . 33 



THE JOURNAL OP EXPERIMENTAL lOOLOGT, VOL. 10, NO. 4. 



