Principles of Heredity 195 



(a) These cross-breds may produce pure D germs of 

 both sexes and pure R germs of both sexes on an 

 average in equal numbers. 



(b) Either the female, or the male, gametes may be 

 alone differentiated according to the allelomorphs, 

 into pure Z>'s, pure jft's, and crosses DR or RD, the 

 gametes of the other sex being homogeneous and 

 neutral in regard to those allelomorphs. 



(c) There may be some neutralisation or cancelling 

 between characters in fertilisation occurring in such 

 a way that the well-known ratios resulted. The 

 absence of and inability to transmit the D character 

 in the RR's, for instance, might have been due 

 not to the original purity of the germs constituting 

 them, but to some condition incidental to or connected 

 with fertilisation. 



It is clear that Mendel realized (b) as a possibility, for 

 he says DR was fertilised with the pure forms to test the 

 composition of its egg-cells, but the reciprocal crosses were 

 made to test the composition of the pollen of the hybrids. 

 Readers familiar with the literature will know that both 

 Gartner and Wichura had in many instances shown that 

 the offspring of crosses in the form (a x 6) ? x c $ were less 

 variable than those of crosses in the form a ? x (b x c) <$ , 

 &c. This important fact in many cases is observed, and 

 points to differentiation of characters occurring frequently 

 among the male gametes when it does not occur or is much 

 less marked among the maternal gametes. Mendel of 

 course knew this, and proceeded to test for such a possi- 

 bility, finding by the result that differentiation was the 

 same in the gametes of both sexes*. 



* See Wichura (46), pp. 55-6. 



132 



