68 Experimental Zoology 
assumption how this numerical relation could be explained. 
When the male, A, and the female, B, germ-cells unite, every 
cell of the hybrid will contain both A and B; in which case 
one dominates, namely, A, giving A(B). If we assume that in 
the germ-cells of the hybrid the characters A and B separate 
again, and go to different cells, half of the germ-cells will con- 
tain the one character only, and the other half the other char- 
acter. This is supposed to take place both in the male and in 
the female individual. The male germ-cells containing A may 
meet egg-cells containing A or B, and conversely the male-cells 
B may meet egg-cells containing A or B. The possible com- 
binations that result are shown in the following diagram : — 
Lass | 
AYNB 
The chances are that, on the average, A will meet B twice as often 
as A meets A, or that B meets B. Hence the combination A(B) 
will occur twice as often as AA or BB. The outcome will be 
1 AA, 2 A(B), 1 BB. Thus according to the assumption of 
two kinds of germ-cells in the hybrid the numerical results agree 
with the actual results of the experiments. For this reason 
Mendel’s assumption of two kinds of gametes has been generally 
accepted. Furthermore the theory can be tested in several 
ways, as will be shown later, and it has so far, on the whole, 
stood the test. When in addition to this it was found that in 
the germ-cells a mechanism exists that seemed capable of carry- 
ing out the postulated process of purification, it appeared to a 
number of modern zodlogists that Mendel’s assumption of two 
kinds of germ-cells in hybrids of this sort is a real and not a 
fictitious explanation of the results. 
An actual example may make clearer Mendel’s principle and 
its interpretation. If a gray house mouse, A, is crossed with a 
white albino mouse, B, the offspring, (F,), will be all gray like 
the house mouse. If these gray hybrids' (F,) are bred to each 
* Following Mendel the cross between two races or varieties is called a hybrid, 
although this term has been usually employed for crosses between species. 
