The Cellular Basis 171 



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

 only special cases of Mendelian inheritance and the full develop- 

 ment of the male or female condition is dependent upon the 

 predominance of male-determining, or of female-determining 

 factors, both hereditary and environmental; while the condition 

 of "intersexes" is the result of the lack of such predominance. 



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 may cause it to go one way or the other ; every or- 

 ganism 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 

 between successive generations. But in addition to these specific 

 facts there are certain general considerations which need to be 

 emphasized. 



I. THE SPECIFICITY OF GERM CELLS 



The conclusion is inevitable that the germ cells of different 

 species and even those of different individuals are not all alike. 

 Every individual difference between organisms must be due to 

 one or more differentiating causes or factors. Specific results 

 come only from specific causes. These causes may be found in the 

 organization of the germ cells or in environmental stimuli, i.e., 

 they may be intrinsic or extrinsic, but as a matter of fact experi- 

 ence has shown that they are generally intrinsic in the germ. In 



