HEREDITY OF SEX 375 



the remaining chromosomes are left white, and for the 

 sake of simplicity only two pairs of the latter are 

 indicated in the somatic cells. 



[Cases like that of Protenor, in which the male has 

 one ' sex-chromosome ' and the female two, are now 

 known in a large number of animals for example, in 

 many Insects, in certain Worms, Molluscs, etc., and in 

 several Mammals, including (probably) Man.] The 

 facts render it scarcely possible to doubt that there 

 is a causal relationship between the characteristics of 

 the female sex and the presence of two heterotropic 

 chromosomes, and that a similar connection exists 

 between maleness and the presence of only one. 



[Several explanations of the mechanism of sex-deter- 

 mination by means of these ' sex-chromosomes ' have 

 been suggested. It was originally supposed that the 

 two chromosomes of the female, though alike in appear- 

 ance, were different physiologically, one bearing a 

 factor for femaleness and the other for maleness, and 

 that the female character was dominant. The hypo- 

 thesis now most generally accepted, however, is that 

 the sex-chromosomes are all alike, and that the differ- 

 ence between the two sexes is produced by the presence 

 of two sex-chromosomes in the female and one in the 

 male. The case would then be comparable with that 

 of a pair of Mendelian allelomorphs in which the 

 dominant character represents the presence of some- 

 thing which is absent in the recessive. In cases of the 

 kind here considered, femaleness is due to the presence 

 of a determiner (the extra chromosome) which is 



