2. This mode of sex determination holds only for those animals in which there is one type of egg and two types of spermatozoa. Experimental evidence furnished first by Doncaster in 1908 on a moth, _Abraxas_, indicated that a number of other forms exists in which matters are reversed, inasmuch as there are two types of eggs and one type of spermatozoa. This condition of affairs exists not only in the moth _Abraxas_, but also in the fowl as shown by Pearl. In these forms it is assumed that all the spermatozoa have one sex chromosome X, while there are two types of eggs, one possessing the sex chromosome X, the other possessing Y. When a spermatozoön enters an egg with an X chromosome, the egg will give rise to a male, while if it enters a Y egg, a female will arise. The evidence pointing toward this result is chiefly contained in experiments on sex-limited or more correctly sex-linked heredity; _i. e._, a form of heredity which follows the sex in a peculiar way. Thus colour-blindness is a case of sex-linked inheritance, since this abnormality appears overwhelmingly in the male offspring of a colour-blind person. Doncaster crossed two varieties of _Abraxas_ differing in one character which was sex-linked, and the results of his crossings indicated that in this form there are two types of eggs and one type of spermatozoa.[184] [184] Doncaster, L., _The Determination of Sex_. Cambridge, 1914. These observations on sex-linked heredity confirm the idea that the sex chromosomes determine the sex. The most extensive and conclusive experiments along this line are those by Morgan on the fruit fly _Drosophila_. In this form there are two kinds of spermatozoa and one kind of eggs; the egg has one X chromosome, while one-half of the spermatozoa has an X the other a Y chromosome; the entrance of the latter into an egg gives rise to a male, of the former to a female. While the eyes of the wild fruit fly _Drosophila ampelophila_ are red, Morgan[185] noticed in one of his cultures a male that had white eyes. This white-eyed male was mated to a red-eyed female. The offspring, the F₁ generation, were all red eyed, males as well as females. These were inbred and now gave in the F₂ generation the following three types of offspring: [185] Morgan, T. H., _Heredity and Sex_. New York, 1913. (1) 50 per cent. females, all with red eyes. (2) 50 per cent. males { 25 per cent. with red eyes. { 25 per cent. with white eyes.