(1) and (2) give females, both red eyed, since both contain a red-factored =X= chromosome. (3) and (4) give males, (3) giving rise to red-eyed males, since it contains a red-factored =X= chromosome, (4) producing males with white eyes since this X chromosome is lacking the factor for red eyes. Since all four combinations must appear in equal numbers (provided the experimental material is ample enough, which was the case in these experiments), in the F₂ generation both males and females should have red eyes and in the F₂ generation all the females should have red eyes and half of the males should have red, half white eyes. These results were obtained. The experiments were carried further. No white-eyed females had appeared thus far. On the same assumptions of the relation of the =X=, X, and Y chromosomes to the heredity of sex as well as to eye colour it was possible to predict under what conditions and in which proportions white-eyed females should arise. Thus if a red-eyed female of the F₂ generation (a cross between white-eyed male and normal female) be mated with a white-eyed male the result should be an equal number of white-eyed males and white-eyed females if the chromosome theory of sex determination were correct. The reasoning would be as follows: The red-eyed female, having the chromosome constitution =X=X should form two kinds of eggs in equal numbers with the constitution =X= and X; the white-eyed male having the chromosome constitution XY should form two kinds of spermatozoa X and Y. The following four types of individuals must then be produced in equal numbers: (1) =X=X, (2) XX, (3) =X=Y, and (4) XY. In this case (2) must give rise to white-eyed females and (4) to white-eyed males, while (1) must give rise to red-eyed females and (3) to red-eyed males. Hence white-eyed males and females and red-eyed males and females are to be expected in this case in equal numbers, and this was actually observed. The numerical agreement in this and the other experiments between the expected and observed result cannot well be an accident. The fact that the inheritance of sex-linked characters in man follows the same laws as in _Drosophila_ is a strong argument in favour of the assumption that in man, also, sex is determined by two kinds of spermatozoa. Morgan and his students discovered no less than thirty-six sex-linked characters in _Drosophila_, and each behaved in a similar way to the red and white eye colour in regard to sex-linked inheritance, so that the chromosome theory of sex determination rests on a safe basis. That sex is merely determined by the number of X chromosomes, not by the Y chromosome, is proved by the facts that the Y chromosome may be completely absent as in _Protenor_ and that Bridges[186] has found a type of female _Drosophila_ with a chromosome formula XXY whose sex was not affected by the supernumerary Y. [186] Bridges, C. B., _Genetics_, 1916, i., 1.