1 68 Heredity and Environment 



In such cases one portion of the body shows male characters, an- 

 other portion female ones and a study of the chromosomes in 

 these regions shows that in the former the male combination of 

 chromosomes is present, in the latter the female combination. 

 This comes as near to a demonstration of the truth of the chro- 

 mosomal theory of sex determination as is possible. 



In either sex many secondary sexual characters of the other 

 sex are present during development and traces of these may per- 

 sist in the adult; but one set of these characters develops fully 

 in the male and another set in the female, so that they may be 

 called sex-limited. The development of the secondary sex char- 

 acters is usually determined by internal secretions from the ova- 

 ries or testes, though in some cases they may develop after these 

 ( organs have been removed, but in the last analysis both primary 

 and secondary sex characters are dependent upon the sex deter- 

 miners in the germ cells. Sex and sex-limited inheritance are 

 only special cases of Mendelian inheritance in which conditions 

 of dominance differ in the two sexes, depending upon whether 

 the factor for sex is simplex or duplex. 



On one point there is general agreement, namely every organ- 

 ism is at the beginning of ontogeny so evenly balanced between 

 maleness and femaleness that very slight changes in heredity or 

 environment cause it to go one way or the other ; every organism is 

 potentially both male and female, and even in the fully developed 

 state each sex carries the vestiges of suppressed organs of the 

 other sex. 



D. THE MECHANISM OF HEREDITY 



The mechanism of heredity, as contrasted with the mechanism 

 of development, consists in the formation of particular kinds of 

 germ cells and in the union of certain of these cells in fertiliza- 

 tion. We have briefly traced the origin, maturation and union of 

 male and female sex cells in a number of animals, and in these 

 phenomena we have the mechanism of the hereditary continuity 



