Professor T. H. Morgan has made numerous observations and experiments on a single culture of the fruit-fly, _Drosophila ampelophila_, bred in bottles in the laboratory for five or six years. He has not only studied the chromosomes in the gametes of this fly, and made Mendelian crosses with it, but has obtained numerous mutations, so that his work is a very important contribution to the mutation doctrine. Drosophila in the hands of Professor Morgan and his students and colleagues has thus become as classical a type as Oenothera in those of the botanical mutationists. Different branches of Morgan's work are discussed elsewhere in this volume, but here we are concerned only with its bearing on the question of the determination of sex. He describes [Footnote: _A Critique of the Theory of Evolution_. Princeton University Press and Oxford University Press, 1916.] the chromosomes of Drosophila as consisting in the diploid condition of four pairs, that is to say, pairs which separate in the reduction division so that the gamete contains four single chromosomes, one of each pair. In two of these pairs the chromosomes are elongated and shaped like boomerangs, in the third they are small, round granules, and the fourth pair are the sex-chromosomes: in the female these last are straight rods, in the male one is straight as in the female, the other is bent. The straight ones are called the X chromosomes, the bent one the Y chromosome. The fertilisations are thus XX which develops into a female fly, and XY which develops into a male. Drosophila therefore is an example of one of the cases described by Wilson. Dr. Wilson (_loc. cit._) discusses the question of how we are to interpret these facts, in particular, the fact that the X chromosome in fertilisation gives rise to females. He remarks that the X chromosome must be a male-determining factor since in many cases it is the only sex-chromosome in the males, yet its introduction into the egg establishes the _female_ condition. This is the same difficulty which I pointed out above in connection with the Mendelian theory that the female was heterozygous and the male homozygous for sex. Dr. Wilson points out that in the bee, where fertilised eggs develop into females and unfertilised into males, we should have to assume that the _X_ chromosome in the female gamete is a female determiner which meets a recessive male determiner in the _X_ chromosomes of the sperm. When reduction occurs, the _X_[female] must be eliminated since the reduced egg develops always into a male. But on fertilisation, since the fertilised egg develops into a female, a dominant _X_[female] must come from the sperm, so that our first assumption contradicts itself.