male egg. Hence, since the female element dominates in these animals when the two sexes meet, the individual must become a female. Since therefore such an egg carries the male element in a recessive form, this element may, if it becomes separated from its female associate, give rise to a male. In this way the theoretical difficulty referred to above is met.
Let us follow out a little further the applications of this view. It is probable in the honey-bee that all the eggs give off two polar bodies. Consequently unfertilized eggs must produce pure males. If they are fertilized by female spermatozoa they will give rise to females, and, on the hypothesis, only female spermatozoa can enter male eggs.
In rotifers and certain crustaceans only one polar body is given off, but since this is the first polar body it does not involve the question of sex. Consequently the parthenogenetic eggs in these forms are sex hybrids. If at any time the conditions change so that one or the other sex element dominates, males and females may arise. But why the female element should dominate in some eggs and the male in others is not explained, and thus we are in exactly the same predicament as we were before Castle's hypothesis was proposed.
One case of special difficulty should not pass unnoticed since Castle has made an interesting suggestion that appears to clear up a difficulty, provided the facts on which the conclusion rests are confirmed. The eggs of the honey-bee extrude two polar bodies, as we have said, and hence are purely male. It is assumed that these males must produce spermatozoa that are female. This is a necessary assumption, because the eggs of the bee having extruded their two polar bodies are purely male, but become female after they are fertilized. Therefore the spermatozoon must bring the female element into the egg. Castle tries to meet this difficulty of the formation of female spermatozoa in a purely male individual by reference to a recent observation of Petrunkewitsch, namely, that the reproductive organ of the male bee develops not from the egg itself, but from the second polar body which fusing with one of the first pair reenters the egg. This second polar body is, on Castle's theory, purely female, hence the spermatozoa must be female. The ingenuity of the explanation is admirable, and rescues the theory from a fatal objection, but of course even if Petrunkewitsch's results are accurate (and they are not above suspicion) it by no means follows that the spermatozoa that come from the second polar body are female, as Castle assumes. The facts in regard to the parthenogenesis, and in regard to the special case of the bee, may possibly be given a much simpler explanation than that which Castle applies to them. For instance, if in certain insects the addition of the chromatin material of a spermatozoon (or what amounts to the same thing the chromatin contained in a polar body) determines that the egg shall
