130 THE BIOLOGY OF INSECTS 



eyed males and red-eyed females ; sometimes this " criss- 

 cross " inheritance fails to work in some 5 per cent, of the 

 population which appear just like their parents — red-eyed 

 males and white-eyed females. C. B. Bridges (1916), by 

 careful breeding observations confirmed by microscopical 

 study of the cell-nuclei, has demonstrated that these 

 exceptions arise from *' non- disjunction " of the sex- 

 chromosomes in the maturation divisions of the egg-nuclei ; 

 the members of a pair of white-bearing, female- determining 

 (x) chromosomes may not part company with each other 

 but may both pass into the same daughter- nucleus, so that 

 a ripe egg, instead of carrying the normal one x chromosome 

 may carry two or none. Bridges has proved that if the 

 former kind of egg be fertilised by a sperm with the male 

 factor the offspring will be the exceptional white-eyed 

 female, while if the egg without any sex- chromosome be 

 fertilised by a sperm v^th the female and red-eye factors 

 the offspring will be the exceptional red-eyed male. These 

 conclusions are startling when compared with the con- 

 ditions that apparently determine sex in the normal results 

 of cross-breeding already considered (pp. 128-9). In 

 these exceptional families the white-eyed flies are females 

 although there is a male-factor (which is commonly regarded 

 as " dominant ") in the zygote nucleus. But then there are 

 two female factors, so that we might conclude that the 

 quantity of the respective determinant in the nucleus helps 

 to decide the resulting sex ; the single male-factor may be 

 dominant over one female factor but has to give way to a 

 " double dose " of femininity ! The constitution of the 

 exceptional red-eyed male, however, is still more surprising, 

 for this insect has no chromosome with the male factor ; 

 his only sex-chromosome is female-producing. In view of 

 these facts several students of these subjects, including 

 R. Goldschmidt in his recent (1923) critical discussion of 

 the question of sex- determination, believe that the normal 

 heterozygotic male creature (fM or xy) is a male, not 

 because he possesses the *' male " factor (y), but because 

 he has only one chromosome v^dth the " female '* factor (x). 



