208 MENDELISM 



The most important phenomenon of all, however, 

 is that which is found to occur at the formation of the 

 germ cells of the heterozgyote plant or animal. What- 

 ever the appearance of the hybrid form may have been, 

 at this stage in its history the determining factors for 

 each member of the pair of parental allelomorphs 

 reappear in their entirety in certain cells which by 

 their diversion give rise to the gametes, and at one 

 of the divisions in question the parental characters (in 

 a potential condition) separate completely from one 

 another, so that half the gametes bear one allelomorph 

 and half of them the other. In cases where more than 

 one pair of allelomorphs has taken part in the cross, the 

 members of each pair are found, as a rule, to undergo 

 this process of segregation quite independently of all 

 the other pairs. 



Whether this process of Mendelian segregation is a 

 universal one, we cannot tell at present, but we do 

 know that it is of very widespread occurrence. Nor 

 do we know whether a similar process takes place at 

 the gamete-formation of homozygotes, though it seems 

 scarcely possible to suppose otherwise. 



We have seen enough to enable us to recognise 

 very clearly the vital importance of an understanding 

 of the constitution of the gametes in all questions of 

 heredity. There must exist in the gametes, in an 

 uncombined condition, those units which by their 

 combination in zygotic organisms lead to the appear- 

 ance of the characters which we can recognise. But 

 we have seen that, owing to the appearance of domi- 

 nance and other kindred phenomena, the visible external 



