Experimental Hybridizing 77 



explanation of the assumed purity of the germ-cells in the Men- 

 delian cases, yet more careful consideration shows that in order 

 to do so certain assumptions are necessary that are not above 

 suspicion. 



It may be seriously questioned, I think, whether the germ-cells 

 of Mendelian hybrids are pure. It is true that the Mendelian 

 proportion, 1:2:1, in the second (F%) generation can be most 

 easily accounted for by assuming two kinds of male and 

 two kinds of female germ-cells, each kind existing in equal 

 numbers ; but the assumption that the two kinds must be pure 

 germ-cells meets with serious difficulties when certain results 

 are considered. It will suffice to point out here that the main 

 difficulty lies in the behavior of the 'so-called extracted reces- 

 sives which ought to be a pure strain on the assumption of 

 "pure" germ-cells, but which have been shown on the con- 

 trary to contain in a latent condition the dominant character. 

 I have tried to show that the results may still be accounted 

 for even if the germ-cells of the hybrids of generation (FJ 

 are not pure in regard to any pair of contrasted characters, 

 such as gray and white, but that both characters are present 

 in all the germ-cells. The two kinds of germ-cells that the 

 hypothesis calls for may be referred to the alternating 

 dominance in the germ-cells of each of the two contrasting 

 characters. The Mendelian proportion can be accounted for 

 on this assumption as well as on the accepted interpretation 

 of pure germ-cells, and the latency of the dominant char- 

 acter in the extracted recessive can also be explained on my 

 view, but not on the other. . An example may make my 

 meaning clearer. 



Suppose a white and a gray mouse are paired. The germ- 

 cells of the white mouse are white-producing, or briefly white, 

 those of the gray mouse are gray-producing, or briefly gray. The 

 fertilized egg will contain both characters, and since the gray 

 dominates the white, the symbol G(W) will represent the con- 

 dition in the mouse itself. In its germ-cells both the gray, G, 

 and the white, W, exist, presumably combined in some way. 



