become a female, we can explain the results without complicating the problem by the assumption of male and female spermatozoa, and male and female eggs. Castle's theory appears needlessly complex, and the whole attempt to apply the Mendelian principle to the question of sex does not appear to have been very successful. The weakest side of the theory has already been spoken of, namely, that it fails to account for the very problem that a theory of sex should explain, namely, the problem of what it is that determines whether an egg that contains both potentialities becomes a male or a female.
The reaction that has set in against the old view, that the sex of the embryo could be determined at a relatively late stage in development, is no doubt in the right direction. It has been shown in several cases by recent discoveries that the sex of the embryo is already determined in the fertilized egg, and in other cases it appears to be determined even before fertilization, but this need not mean that there are male and female eggs, and male and female spermatozoa. We have just examined two recent theories that rest on assumptions of this kind and have found, in my opinion, that they are both unsatisfactory. Let us see whether it may not be possible to bring under one point of view the old and the new discoveries in regard to the determination of sex, and construct a hypothesis that does not involve the idea that there is separation of the primoidia of sex in the germ-cells.
1. It has been shown in a few cases that two kinds of eggs are produced which become male and female individuals, in some cases with, in others without, fertilization. It may be erroneous to conclude from these facts that the eggs themselves are male and female in the sense that the elements (primoidia) that determine the sex of the embryo have become separated and confined to male or to female eggs. In a case like that of the silk-worm, where a graded series exists, the size of the egg appears to be the determining factor in respect to which sex develops, not that the female sex-elements are found only in the large eggs, and the male elements in the small eggs. It seems more reasonable to assume on the contrary that both elements are present in all kinds of eggs. In other cases other factors than that of size determine which sex develops.
In regard to the two forms of spermatozoa that have been found in a few species, there is no evidence that one sort contains only the primoidia of a male individual and the other kind those of the female. In those arthropods in which an accessory chromosome has been found we have no evidence to show that this chromosome is the male or the female element, and so long as we know nothing at all in respect to the conditions in the egg it is useless to speculate further on these cases.
