572 I SCIENCE PROGRESS 



the cross between male and female gives typically a result 

 (approximate equality of the two parental forms) that is the 

 same as that of the cross between a heterozygote and a 

 homozygote recessive. That this is something more than a 

 mere coincidence is rendered probable by direct experiment, 

 notably by Correns's remarkable results with reciprocal crosses 

 between different species of dioecious plants, which lead him 

 to conclude; that one sex is in fact homozygous in respect to 

 the sexual characters, the other heterozygous. 



All this, and much more in the same direction, unmistakably 

 indicates that sex is determined by a mechanism that pre- 

 exists in the germ-cells. The most direct and obvious evidence 

 of this is given by the cytological observations to which 

 attention will be directed in this review. These observations 

 establish the fact that in certain animals (including many air- 

 breathing arthropods, and one or two echinoderms and nema- 

 todes) the sexes differ visibly in respect to the constitution 

 of the cell-nuclei. They show, further, that these nuclear 

 differences must be established at the time the eggs are 

 fertilised, and are traceable to original differences in the 

 gamete-nuclei before fertilisation takes place. The somewhat 

 complicated data on which this conclusion rests may be more 

 readily understood after a brief preliminary statement of the 

 most essential facts, beginning with the group best known in 

 this regard, the insects. 



In many species of insects there are two kinds of spermatozoa, 

 equal in number, which in the early stages of their development 

 differ visibly in respect to nuclear constitution ; while there 

 is but one kind of egg, which is in nuclear type identical 

 with one of the classes of spermatozoa. That is to say, if 

 the two kinds of spermatozoa be designated as the " X-class " 

 and the " Y-class," respectively, the eggs are all of the X-class. 

 The male may, accordingly, be designated as the heterogametic 

 sex, the female as the homogametic. The evidence demonstrates 

 (though at one point it still remains indirect) that fertilisation 

 of the egg by the X-class of sperm produces females, by the Y-class 

 males. General formulas for sex-production in these animals 

 may accordingly be written : 



X + X = Female and X + Y = Male, 

 which is identical in form with the Mendelian formulas 



R + R = RR and D + R = DR. 



