AND MENDEL’S LAW 265 
letters. A and a are the allelomorphs of one pair, 
B and b those of the other, and we will suppose that 
one of the parents exhibits the characters A and b 
and the other the characters a and B (Fig. 44). Then, 
in the zygote resulting from fertilization, A, a, B, aut b 
will all be present. 
Since all the cells, at least in the direct line of 
ancestry of the gametes, must contain every allelo- 
morph, it will be necessary for the particle representing 
each allelomorph always to divide into two before a 
cell division takes place, for only in this way can some- 
thing corresponding to each allelomorph pass into each 
of the two cells produced by the division. And a 
similar process will be repeated at each somatic 
mitosis (Fig. 44). At the formation of the germ-cells, 
however, or at some preceding cell division, the two 
members of each pair of allelomorphs must become 
separated from one another in such a way that the 
particles originally derived from different parents 
pass over into different cells. When two pairs of 
allelomorphs are concerned, this process of separation 
can take place in either of the two ways shown in 
Fig. 45. And the experimental evidence shows that 
the two methods occur with equal frequency in the 
formation, of the germ-cells of the same heterozygote. 
Anyone who has succeeded in following the above 
account of the behaviour of the supposed particles 
representing Mendelian allelomorphs in the cells of a 
hybrid organism, on comparing it with the preceding 
description of the behaviour of chromosomes in the 
