no POPULAR SCIENCE MONTHLY. 



other two male. Similarly for the spermatozoa; two of each four 

 (formed from the first spermatocyte) are female, and two are male. 



The evidence on which Castle rests his assumption that there are 

 two kinds of spermatozoa, as well as two kinds of eggs, is contained 

 in the following statement: "That sex is borne by the egg is shown 

 clearly by the case of parthenogenetic animals, which without the 

 intervention of a male produce young of both sexes. That the sper- 

 matozoon also bears sex is manifest in the case of animals like the 

 honey bee, for the egg of the bee, if unfertilized, invariably develops 

 into a male, but if fertilized into a female." The finality of the con- 

 clusions drawn from these facts is by no means above question. 



Perhaps the most distinctive part of Castle's paper is his attempt 

 to apply the much-discussed Mendel's law to problems of sex-determi- 

 nation; an idea that had suggested itself to Bateson and Saunders, 

 but had been rejected, because the 'distribution of sex among first 

 crosses shows great disparity from the normal proportions.' Castle 

 does not admit however the force of this objection. 



A specific example may be the simplest way of illustrating Mendel's 

 law and its application to sex as maintained by Castle. If a white 

 mouse is crossed with a wild gray mouse all the offspring of this 

 first cross will be gray like the wild mouse. The gray color of the 

 gray mouse is said to be dominant and the white color (inherited from 

 the other parent) does not appear, but is supposed to be present in a 

 sort of latent condition. It is said to be recessive. If now these 

 primary hybrid mice are interbred some of their young will be white 

 and the rest gray in the proportion of one to three. If these white 

 mice, when they become grown, are interbred their offspring will 

 always be white as well as all their subsequent descendants. Some 

 of the gray mice will also breed true, but the rest that are gray hybrids 

 will, if interbred, give rise to some white and some gray in the propor- 

 tion again of one to three. This is only a partial statement of 

 Mendel's law, but will suffice for our present purposes. 



The explanation that Mendel offered to account for the proportion- 

 ate number of individuals that inherit the dominant and the recessive 

 characters is very simple and is probably correct. As applied to our 

 illustration of the mice it would be as follows: When the egg of the 

 white mouse is fertilized by the spermatozoon of the gray mouse the 

 fertilized egg and all the cells into which it divides contain chromatin 

 material in the nucleus half from the white and half from the gray 

 parent. The dark element dominates whenever the two are together, 

 hence the first generation of hybrids are all dark. The cells of this 

 primary hybrid that have gone into the reproductive organs (in the 

 female into the ovary and in the male into the testis) are supposed to 

 be at first like all the other cells of the body, and contain both white 



