TEE DETERMINATION OF SEX. m 



and dark elements.* But at some time in their later history, and 

 presumably at the time when the egg sends ofiE its polar bodies and at 

 the time when the four spermatozoa are formed, a separation of the 

 dark from the white elements occurs, so that two cells of the one kind 

 and two of the other are formed. Thus the germ-cells are, as it were, 

 purified, and consist of those that contain only white and of those that 

 contain only dark elements. This is supposed to be the condition of 

 the germ-cells in the ovary and in the testis of the primary hybrids. 



Suppose now that these hybrids breed together, the white and the 

 black spermatozoa will meet the white and the black eggs, and since 

 it is a question of chance alone how they will come together, all possible 

 combinations will be made. When a white germ-cell meets with a 

 white one, a white individual results, and since it contains only white 

 elements all its descendants will be white (if it is bred, of course, 

 to white individuals). If a gray germ-cell meets a gray germ-cell, a 

 gray individual will result, and all its purely bred descendants will 

 be gray. If, however, a white and a gray germ-cell unite, the individual 

 that develops will contain both elements in all its body-cells and, since 

 the gray always dominates in such combinations, the individual will be 

 gray, but will have the white as a recessive character that may crop out 

 in subsequent generations. On the theory of chance combinations 

 there will be twice as many of these gray-white individuals as of the 

 white or of the pure gray. The series stands 2:1:1. Since in outward 

 appearance all the gray-white mice are like the pure gray, we get three 

 grays to every one white. 



Let us now return to Castle's theory and see how he tries to make 

 an application of Mendel's principle to sex. Just as there are two 

 kinds of mice in our illustration, white and black, there are two kinds 

 cf sexual individuals, males and females. It is now assumed that 

 the germ-cells, when they reach their final divisions, separate their 

 male from their female elements, giving pure male and pure female 

 eggs, and pure male and pure female spermatozoa. If, as in the 

 mice, all chance combinations of the germ-cells are possible, there will 

 result three kinds of individuals in the proportion of 2:1:1. The first 

 of these, that are twice as common as either of the other two, would 

 be sex-Jiyhrids. If we assume, as in the mice, that one character always 

 dominates in such a combination, the male let us say, there would be 

 twice as many males of this hybrid kind as there are individuals of 

 either of the other two pure kinds, and since there are as many pure 

 males as there are pure females, there would be in all three times as 

 many males born as females. Since we know that there is no such 

 disproportion of one sex to the other, it appears absurd to attempt to 



* More accurately, elements corresponding to the white and black colors. 



■^ 



"% 



