114 POPULAR SCIENCE MONTHLY. 



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 develop- 

 ment, 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 deter- 

 mined in the fertilized egg, and in other cases it appears to be deter- 

 mined 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 pro- 

 duced 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. 



