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, and 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 



