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 (F) 
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. 
