2. There is experimental evidence pointing to the conclusion that factors, external to the egg itself, may determine in some species what kind of eggs will be produced, as in Hydatina, and in the aphids where a change in the food causes the appearance of males and females. In bees the addition of the chromatin of the spermatozoon appears as a rule to determine that the egg gives rise to a female.
In other cases it appears that the addition of the chromatin in one of the polar bodies may accomplish the same result. Here the relation may be purely a quantitative one. In other animals the addition of the spermatozoon to the egg is not, it appears, the factor that determines the sex.
3. It is known in bees and in butterflies that individuals sometimes appear that are male on one side of the body and female on the other side. The explanation of this peculiarity may be found in the unusual way in which the nucleus of the fertilized egg is divided. If, for instance, all or most of the chromatin brought in by the spermatozoon should be carried into one of the first formed cells along with half of the chromatin of the egg-nucleus, then all the cells that descend from this cell may develop female characters, and all those from the other, male characters. This need not mean that the spermatozoon has brought into the egg female characters that dominate in all the cells in which it is contained, but only that those cells that contain more of the chromatin differentiate their female characters, and all those cells that contain less chromatin differentiate their male characters only.
4. Having discovered that the sex is already determined in the unfertilized egg in some cases, and in others that it is connected with the process of fertilization, the question at once suggests itself whether the determining influence comes from the nucleus or from the cytoplasm. At present we have no conclusive evidence pointing in either direction. That the quantity of the nuclear material may be important seems probable in the case of the bee. That the size of the egg, which is due to a greater amount of cytoplasm, may be a factor in the result seems in other cases to be important, but so long as we do not know what relation the nucleus bears to the cytoplasm in these forms we can not decide as to the meaning of greater volume as a sex determinant. If, as seems highly probable, identical twins come from halves of the same egg, then since the pairs may be of either sex it seems to follow that the absolute size of the egg is not a factor. Whether in these cases the relative amount of chromatin in the nucleus enters into the problem remains to be shown.
It should be pointed out that while we must suppose that the influences in the embryo that control the development of one or of the other sex reside, or have resided, in the nucleus of the egg, this is a
