THE CELLULAR BASIS 161 



application, that it ought not to be abandoned 

 until unmistakable evidence can be adduced 

 against it. For the present at least we are 

 justified in concluding that sex is irrevocably 

 determined at the time of fertilization. 



C. 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 fertilization. We have briefly traced the 

 origin, maturation and union of male and fe- 

 male 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 dif- 

 ferent individuals are not all alike. Every in- 

 dividual difference between organisms must 



