DETERMINATION OF SEX DONCASTER. 479 



numerous papers on spermatogenesis and oogenesis, which have led 

 to the hypothesis that the sex determinant is a visible chromosome- 

 like body, would occupy more space than is available, so we will take 

 the work of E. B. Wilson as typical, although similar phenomena had 

 been 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 (idio- 

 chromosomes) 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 re- 

 ducing 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 regards their chromo- 

 some 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 8 

 he modified this hypothesis in favor of one which will allow the 

 sex determinants to be regarded as Mendelian characters, femaleness 

 being dominant over maleness. The two idiochromosomes 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 fertilization; so that a male-bearing sperm 

 can conjugate only with a female-bearing egg and a sperm bearing 

 no sex determinant (idiochromosome) with a male-bearing egg. If 

 femaleness is dominant, all fertilized eggs having two idiochromo- 

 somes will become females, those having only one, males. It is inter- 

 esting that breeding experiments with Lepidoptera, which will be 

 mentioned below, led the present writer 3 to formulate an almost 

 exactly similar hypothesis at almost the same time. But it will be 

 seen that later experiments with moths suggest that a slightly differ- 

 ent explanation of the facts is possible. 



1 " Studios on Chromosomes," i, ii, iii, and iv, Journ. Exp. Zoo. 1905, 1906, 1909. 

 Also several papers in Science, 1905-7, etc., especially 1909, vol. 29, 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. Journ. Anat., 

 vol. 7, 1907, p. 279. 



2 " Studies on Chromosomes," iii, .Tourn. Exp. Zoo., vol. ?>, 1906. No. 1. For a still 

 later suggestion of Wilson's see footnote near the end of this article. 



3 Doncaster and Raynor, Proc. Zoo. Soc. 1906, vol. 1, p. 125. 



