Page:Popular Science Monthly Volume 64.djvu/114

110 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 spermatozoon 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 conclusions 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-determination; 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 proportion 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 proportionate 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