THEORIES OF HEREDITY 89 



V. DETERMINATION OF SEX. 



The fundamental distinction between the two sexes is 

 the possession of the sex-gland, the male producing sperm- 

 cells, while the female has an ovary with egg-cells. All 

 the other distinctive sex characteristics, as the organs for 

 copulation, or, in the case of the female, those for bearing 

 and suckling the young, are secondary. We have already 

 seen in a previous chapter that the separation of the sexes 

 is by no means absolute. Not only can both sexes in certain 

 species be united in a hermaphrodite individual, but we 

 have seen that even the highest animals pass through an 

 embryonic phase where both kinds of sex-organs are still 

 existent. It is only at a later stage of the development 

 that the sex is finally determined, and this occurs the 

 earlier the higher the organism stands in the scale of life. It 

 follows from this that each being is potentially of either sex. 

 Weismann, therefore, assumes that the germ-plasm contains 

 a double set of determinants male and female and that 

 characteristics of either sex come to expression according 

 as the male or female determinants have the deciding influ- 

 ence. By this theory of double sex determinants it is ex- 

 plainable how the transmission of sex characters through 

 the opposite sex is possible, as when the characteristics 

 of the grandfather (beard, etc.) reappear in the son of his 

 daughter qualities which must have lain dormant in the 

 daughter. In castration, where the sex-glands are re- 

 moved, the development of the usual sex characters is 

 inhibited, and the latent characters of the opposite sex 

 become apparent. 



But we are still faced with the fundamental question : 

 What decides the ultimate predominance of one sex over 

 the other ? At what stage does the determination of sex 

 take place ? We must here point out that it is wrong to 

 regard the ovum, as is often done, as a female, and the 

 spermatozoon as a male, product of the body. Both are, 

 as we have already explained, not complementary to each 



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