THE DETERMINATION OF SEX. 115 



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 deter- 

 mines 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 cyto- 

 plasm. At present we have no conclusive evidence pointing in either 

 direction. That the quantity of the nuclear material may be im- 

 portant 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 deter- 

 minant. 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 



