106 THE AMERICAN NATURALIST [Vol. XLIX 



To find out whether these flies were actually sterile, 

 each of the 14 remaining pairs — 9 having meanwhile 

 died — were separated and every individual mated to wild 

 red-eyed stock. The sterility of the pink flies, both male 

 and female, was found to be absolute, while all red of 

 both sexes were fertile. 



As an additional test, some of the offspring of the last 

 cross were inbred en masse in order to extract the pink 

 flies which they would produce, since some of them were 

 heterozygous for eye color. 



A small number of pink flies were obtained and mated 

 to their red brother and sisters: each pink female was 

 put in a bottle with 3 or 4 red males, and each pink male 

 with 3 or 4 red females. Out of 19 individuals thus tested, 

 only three were found to be fertile ; the remaining 16 were 

 sterile. 



These facts seem to indicate that some factor or group 

 of factors which make for sterility were present in the 

 ''pink "-containing gamete. The results are the more 

 significant since the hybrid fly, in which this condition 

 prevailed, produced a very low pink ratio. Of the control 

 in which 10 pairs were found to be heterozygous for eye- 

 color with an expectation of 1:1, the following results 

 were obtained : 



TABLE IX 



Eecord of the Offspring of F 2 Flies of a Cross of Pink $ by Bed c? in 

 Which a Batio Was 3: 1, as Expected. (See pair F of Table VIII) 

 Bed Heterozygous ? by (Brother) Pink <J 



The reciprocal cross gave the following: 



