diagrammatically below, the dark rod representing the _X_ red chromosome, the clear rod the _X_ white chromosome, and the bent clear rod the _Y_ white chromosome. According to Morgan, the heredity of colour-blindness in man is to be explained exactly in the same way as that of white eye in _Drosophila_. A colour-blind man married to a normal (homozygous) woman transmits the peculiarity to half his grandsons and to none of his grand-daughters. Colour-blind women are rare, but in the few cases known where such women have married normal husbands the defect has appeared only in the sons, as in the second of the diagrams below. Parents Red-eyed male White-eyed female XR XR x XW YW F1 Red-eyed male Red-eyed female XR XW XR YW F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female XR XR XW XR XR YW XW YW Homozygous. Heterozygous. Heterozygous. Homozygous. White-eyed male Red-eyed female XW XW x XR YW F1 Red-eyed male White-eyed female XW XR XW YW F2 White-eyed male Red-eyed male White-eyed female Red-eyed female XW XW XR XW XW YW XR YW Homozygous. Heterozygous. Homozygous. Heterozygous. It must be explained that according to this theory the normal male is always heterozygous, because the _Y_ chromosome never carries any other factor except that for sex; it is thus of no more importance than the absence of an _X_ chromosome which occurs in those cases where the male has one sex-chromosome and the female two. According to the researches of von Winiwarter [Footnote: 'Spermatogénèse humaine,' _Arch. de Biol._, xxvii., 1912.] on spermatogenesis in man, the latter is actually the case in the human species. This investigator found that there were 48 chromosomes in the female cell, 47 in the male; after the reduction divisions the unfertilised ova had 24 chromosomes, half the spermatids 24 and half 23, so that sex is determined in man by the spermatozoon. Morgan believes that the heredity of haemophilia (the constitutional defect which prevents the spontaneous cessation of bleeding) follows the same scheme, and also at least some forms of stationary night-blindness-- that is, the inability to see in twilight.