442 SCIENCE PROGRESS 



gous for sex, and that the two X chromosomes carried re- 

 spectively male and female factors, femaleness being dominant. 

 The male was, on the other hand, homozygous, the single X 

 chromosome carrying a male factor. 



Thus, if M and F stand for maleness and femaleness respec- 

 tively, every female has the constitution M F and every male 

 the constitution M O, O denoting the absence of any sex- 

 determiner. Mature eggs would, therefore, be of two kinds, 

 those carrying M and those carrying F ; and two kinds of 

 sperms, M and O carriers. 



At fertilisation four classes of zygotes would be produced, 

 MM, MF, MO, FO, if any class of sperm was capable oif 

 fertilisating any class of egg. As FO and MM would, ex 

 hypothesis, give rise to no known sex, the theory was modified 

 by assuming selective fertilisation, in which only F-bearing eggs 

 could be fertihsed by M-bearing sperms, and M-bearing eggs 

 only by 0-bearing sperms. 



In the Lepidoptera and Birds, on this hypothesis, maleness 

 is dominant to femaleness and the male is of constitution MF, 

 the female FO. 



The great objection to this theory is the assumption of 

 selective fertilisation, for which there is at present no justification. 

 The assumption that the male is heterozygous for sex, and the 

 female homozygous, as Correns ^ has suggested in his theory, 

 overcomes the difficulty of selective fertilisation, for it is obvious 

 that either a male or a female carrying sperm can fertilise 

 the ova, which are, in this case, all alike in having the female 

 constitution. On other grounds, however, the theory must be 

 abandoned. For instance, taking the case of the bee, it is 

 well known that the males are produced from parthenogenetic 

 eggs, which have undergone their reductive divisions. The 

 chromosome number, therefore, is halved or haploid. 



Now, such eggs must contain the male determiner, for 

 otherwise how do drones develop from them ? But, according 

 to the theory, no such determiner is present in such eggs. 



These qualitative hypotheses to account for sex-determina- 

 tion are subject to such grave objections that they are now 

 generally abandoned in favour of the simpler quantitative 

 hypothesis first propounded by Wilson ' and later by Castle.^ 



Here it is suggested that there are not two distinct factors, 

 one for maleness and one for femaleness, but that the sex 

 depends on the quantity of sex-determining substance carried 

 by the X chromosomes. Thus, in the majority of animals 

 the female is quantitatively greater in some substance than 



1 Correns, C, Bestimmung ti. Vererbung des Geschlects. Leipsig, 1907. 

 » Wilson, E. B., Science, 29, 1909, P- 53- 

 3 Castle, W. E., Science, 29, 1909. P- 395- 



