formulas XO, XY, and XYY, and {10} likewise females with the formulas XX, XXY, and XXYY, that the Y is without effect either on the sex or on the visible characters (other than fertility) of the individual. The evidence is equally positive that sex is quantitatively determined by the X chromosome--that two X's determine a female and one a male. For in the case of non-disjunction, a zero or a Y egg fertilized by an X sperm produces a male, while conversely an XX egg fertilized by a Y sperm produces a female. It is thus impossible to assume that the X sperms are normally female-producing because of something else than the X or that the Y sperm produce males for any other reason than that they normally fertilize X eggs. Both the X and the Y sperm have been shown to produce the sex opposite to that which they normally produce when they fertilize eggs that are normal in every respect, except that of their X chromosome content. These facts establish experimentally that sex is determined by the combinations of the X chromosomes, and that the male and female combinations are the causes of sex differentiation and are not simply the results of maleness and femaleness already determined by some other agent. Cytological examination has demonstrated the existence of one XXYY female, and has checked up the occurrence in the proper classes and proportions of the XXY females. Numerous and extensive breeding-tests have been made upon the other points discussed. The evidence leaves no escape from the conclusion that the genetic exceptions are produced as a consequence of the exceptional distribution of the X chromosomes and that the gens for the sex-linked characters are carried by those chromosomes. MUTATION IN DROSOPHILA AMPELOPHILA. The first mutants were found in the spring of 1910. Since then an ever-increasing series of new types has been appearing. An immense number of flies have come under the scrutiny of those who are working in the Zoological Laboratory of Columbia University, and the discovery of so many mutant types is undoubtedly due to this fact. But that mutation is more frequent in _Drosophila ampelophila_ than in some of the other species of _Drosophila_ seems not improbable from an extensive examination of other types. It is true a few mutants have been found in other _Drosophilas_, but relatively few as compared with the number in _D. ampelophila_. Whether _ampelophila_ is more prone to mutate, or whether the conditions under which it is kept are such as to favor this process, we have no knowledge. Several attempts that we have made to produce mutations have led to no conclusive results.