JO GENETICS 



cells (sperm or ova) of this individual. They are alike In 

 the two sexes (except as to their X-chromosomes, of 

 course). 



(4) Now the germ glands in individuals of which the 

 cells contain but one X — in the males — begin to develop in 

 a distinctive fashion: The cells arrange themselves in 

 "cords" and take on a characteristic appearance. At the 

 same time the germ glands of individuals whose cells carry 

 two X's — the females — remain in the indifferent condition. 

 Later, in the males the genital cells divide into small cells, 

 which become the mother cells of the sperms; while in the 

 females they enlarge, later to produce ova. These differ- 

 ences in development are due directly to the fact that one set 

 of cells contains a single X, the other set two X's. 



As seen above, there is a period in which the male has 

 developed distinctive characteristics, while the female is 

 still in the indifferent condition. This turns out to be a mat- 

 ter of importance, as will be brought out later. 



(5) The two sexes continue to develop differently to 

 adult life. The male germ gland becomes the testis, which 

 produces sperms; and the body of the male develops differ- 

 ently from that of the female, producing the male secondary 

 sex characters (mane, beard, greater size, and the like). 

 The female germ gland becomes the ovary, which produces 

 ova; and the body develops the female secondary sex char- 

 acters (mammary glands, diverse body form and size and 

 the like). 



Thus the male and female have become very diverse, dif- 

 fering in a great number of ways, both structurally and phys- 

 iologically. 



All these later differences result in some way from the 

 original difference in the X-chromosomes: from the pres- 

 ence of but one X in one set of individuals, of two X's in the 

 other. Our present question is: through what means does 

 the chromosome difference produce the later differences? 



