22 HELEN DEAN KING 



The first set of records given in table 9 shows the very great 

 difference in the sex tendencies of two sister rats belonging in the 

 B series. Female IIB73 had cast three litters when she de- 

 veloped pneumonia and had to be killed. Each of these litters 

 contained such a large excess of females that among her thirty-two 

 offspring the sex ratio was only 45.4 cf : 100 9 : Female IIB74, 

 on the other hand, showed a very strong tendency to produce 

 male young, whether she was paired with a brother or with a 

 stock male; among her forty-one offspring the sex ratio was 

 162.5 cf : 100 9 . As yet no other sister rats have shown such 

 a pronounced difference in their sex tendencies. 



A very great similarity in the sex tendencies of sister rats is 

 shown by the second set of records in table 9. Each litter cast by 

 17Bi4 and by ITBis contained an excess of female young, whether 

 the sire of the litter was an inbred or a stock male. In each 

 group of litters the sex ratio was about 65 cf : 100 9 . 



Female I2A134 produced an excess of male young in each of the 

 two litters sired by her brother, but the two litters sired by a 

 stock male showed a very great excess of female young. Con- 

 versely, while female I2A135 cast more female than male young 

 when paired with a brother, she showed a strong tendency to pro- 

 duce an excess of male young when mated with a stock male. 



The last set of records in table 9 shows a case where the total 

 number of offspring produced by each of two sister rats contained 

 a nearly equal proportion of the sexes, but this proportion was 

 attained in very different ways. Female I3A45 showed a most 

 pronounced tendency to produce an equal number of male and 

 female young in each of her four litters. In the litters of female 

 ISA/e the sexes were very unequally distributed; one litter of 

 nine young consisted entirely of females — a most unusual phe- 

 nomenon in a litter of such size. 



Numerous other cases, similar to the ones given, could be fur- 

 nished from the records for these inbred rats. The cases cited 

 are sufficient, I think, to show the individual differences in the 

 females regarding their tendencies to cast male or female off- 

 spring. Incidentally, these records show, also, that the female 

 plays a more important role in determining the sex ratio than is 

 generally believed. 



