Ordinarily all the sons and none of the daughters show the recessive sex-linked characters of the mother when the father carries the dominant allelomorph. The peculiarity of non-disjunction is that sometimes a female produces a daughter like herself or a son like the father, although the rest of the offspring are perfectly regular. For example, a vermilion female mated to a wild male produces vermilion sons and wild-type daughters, but rarely also a vermilion daughter or a wild-type son. The production of these exceptions (primary exceptions) by a normal XX female must be due to an aberrant reduction division at which the two X chromosomes fail to disjoin from each other. In consequence both remain in the egg or both pass into the polar body. In the latter case an egg without an X chromosome is produced. Such an egg fertilized by an X sperm produces a male with the constitution XO. These males received their single X from their father and therefore show the father's characters. While these XO males are exceptions to sex-linked inheritance, the characters that they do show are perfectly normal, that is, the miniature or the bar or other sex-linked characters that the XO male has are like those of an XY male, showing that the Y normally has no effect upon the development of these characters. But that the Y does play some positive role is proved by the fact that all the XO males have been found to be absolutely sterile. While the presence of the Y is necessary for the fertility of the male, it has no effect upon sex itself. This is shown even more strikingly by the phenomenon known as secondary non-disjunction. If the two X chromosomes that fail to disjoin remain in the egg, and this egg is fertilized by a Y sperm, an XXY individual results. This is a female which is like her mother in all sex-linked characters (a matroclinous exception), since she received both her X chromosomes from her mother and none from her father. As far as sex is concerned this is a perfectly normal female. The extra Y has no effect upon the appearance of the characters, even in the case of eosin, where the female is much darker than the male. The only effect which the extra Y has is as an extra wheel in the machinery of synapsis and reduction; for, on account of the presence of the Y, both X's of the XXY female are sometimes left within the ripe egg, a process called secondary non-disjunction. In consequence, an XXY female regularly produces exceptions (to the extent of about 4 per cent). A small percentage of reductions are of this XX-Y type; the majority are X-XY. The XY eggs, produced by the X-XY reductions, when fertilized by Y sperm, give XYY males, which show no influence of the extra Y except at synapsis and reduction. By mating an XXY female to an XYY male, XXYY females have been produced and these are perfectly normal in appearance. We may conclude from the fact that visibly indistinguishable males have been produced with the