504 HEREDITY AND SEX 



in all sperms ; but the development of the one or the other sex 

 is determined by some unknown internal relation. . . . The 

 condition that leads to the development of the alternative 

 characters may exist in the e^g alone (as for the male bee), or 

 in the sperm alone (as for certain Hemiptera), or by the com- 

 bination of e^g and sperm, as for the female bee. 



" Admitting that all eggs and all sperms carry the material 

 basis that can produce both the male and female, the two 

 conditions being mutually exclusive when development occurs, 

 the immediate problem of sex-determination resolves itself into 

 a study of the conditions that in each species regulate the deve- 

 lopment of one or the other sex. It seems not improbable that 

 this regulation is different in different species, and that, therefore, 

 it is futile to search for any principle of sex-determination that 

 is universal for all species with separate sexes ; for while the 

 fundamental internal change that stands for the male or the 

 female condition may be the same in all unisexual forms, the 

 factor that determines which of the alternative states is realised 

 may be very different in different species." (1907, pp. 422-4.) 



Man. — Rauber and others have pointed out that the pro- 

 portion of male and female births throughout Europe is fairly 

 constant, the mean being 1060 males to 1000 females, and that 

 the relative constancy of this ratio tells against the view that 

 environmental (notably nutritive) conditions have anything to 

 do with the determination. As we have already mentioned, 

 about 30 per cent, of ordinary twins are of different sexes, while 

 identical (monochorial) twins — surrounded by one foetal mem- 

 brane or chorion, and almost certainly developed from one ovum 

 — are always of identical sex. Both these facts point to the 

 same conclusion, that the sex is determined in the fertilised ovum. 

 Punnett has investigated the proportions of male and female 

 births in different parts of London, and finds that either no 

 influence on the proportion of the sexes can be attributed to 

 parental nutrition, or, at most, only a very small effect. 



