While the genetic evidence forces one to accept crossing-over between the sex chromosomes in the female, that evidence gives no clue as to how such a process is brought about. There are, however, certain facts familiar to the cytologist that furnish a clue as to how such an interchange might take place. When the homologous chromosomes come together at synapsis it has been demonstrated, in some forms at least, that they twist about each other so that one chromosome comes to lie now on the one side now on the other of its partner. If at some points the chromosomes break and the pieces on the same side unite and pass to the same pole of the karyokinetic spindle, the necessary condition for crossing-over will have been fulfilled. THE Y CHROMOSOME AND NON-DISJUNCTION. Following Wilson's nomenclature, we speak of both X and Y as sex chromosomes. Both the cytological and the genetic evidence shows that when two X chromosomes are present a female is produced, when one, a male. This conclusion leaves the Y chromosome without any observed relation to sex-determination, despite the fact that the Y is normally present in every male and is confined to the male line. The question may be asked, and in fact has been asked, why may not the presence of the Y chromosome determine that a male develop and its absence that a female appear? The only answer that has yet been given, outside of the work on _Drosophila_, is that since in some insects there is no Y chromosome, there is no need to make such an assumption. But in _Drosophila_ direct proof that Y has no such function is furnished by the evidence discovered by Bridges in the case of non-disjunction. (Bridges, 1913, 1914, 1916, and unpublished results.) {9}