THE DETERMINATION OF SEX 97 



already observed by Paulmier, McClung, Miss Stevens and 

 others. Wilson, 1 working at a number of genera of Hemipterous 

 insects, finds that in the unreduced germ-cells of the female there 

 are always an even number of chromosomes, two of which 

 (" idiochromosomes") are frequently distinguishable from the 

 remainder by their size. In the males there is either an odd 

 number, owing to the absence of one idiochromosome, or one 

 idiochromosome has the size which it has in the female, 

 while the other is vestigial. At the reducing division the 

 number is halved ; when both idiochromosomes are present 

 they pair together and become separated into different 

 daughter-nuclei ; when in the male there is only one, it passes 

 to one end of the spindle and the other is left without one. 

 In this way it comes about that all the eggs appear alike as 

 regaros their chromosome-groups but in the male there are two 

 kinds of spermatozoa, an idiochromosome being present in the 

 one half but absent or vestigial in the other half. Wilson 

 was therefore led at first to suggest that the spermatozoon 

 determined the sex, sperms with the "accessory" chromosome 

 giving rise to females, those without it to males. Later 2 he 

 modified this hypothesis in favour of one which will allow 

 the sex-determinants to be regarded as Mendelian characters, 

 femaleness being dominant over maleness. The two idio- 

 chromosomes in the female are regarded as male-bearing and 

 female-bearing respectively, so that some eggs after maturation 

 bear maleness, others femaleness. The single idiochromosome 

 in the male is male-bearing and there is supposed to be 

 selective fertilisation ; so that a male-bearing sperm can con- 

 jugate only with a female-bearing egg and a sperm bearing 

 no sex-determinant (idiochromosome) with a male-bearing 

 egg. If femaleness is dominant, all fertilised eggs having two 

 idiochromosomes will become females, those having only 

 one, males. It is interesting that breeding experiments with 

 Lepidoptera, which will be mentioned below, led the present 



1 "Studies on Chromosomes," i. ii. iii. and \v.,Journ. Exp. Zoo. 1905, 1906, 

 1909. Also several papers in Science, 1905-7, etc., especially 1909, vol. xxix. p. 53, 

 a review of the whole subject. It should be mentioned that the accuracy of 

 Wilson's observations has been questioned by several investigators — e.g. Foot and 

 Strobell, Amer. Jonrn. A?iat. vii. 1907, p. 279. 



2 "Studies on Chromosomes," iii., Journ. Exp. Zoo. iii. 1906. No. 1. 

 For a still later suggestion of Wilson's see footnote near the end of this 

 article. 



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