xiv] MENDELIAN CHARACTERS 213 



chromosomes and the appearance of definite somatic char- 

 acters. This evidence is greatly strengthened by arguments 

 of a more indirect kind, based on the extraordinary simi- 

 larity between the known behaviour of the chromosomes in 

 the maturation divisions of the germ-cells and the manner 

 of transmission of Mendelian characters. In the chapters 

 on spermatogenesis and oogenesis, it has been seen that the 

 behaviour of the chromosomes can be summarised as follows. 

 Since the chromosomes of the diploid nucleus before matura- 

 tion are in pairs, the members of each pair being descended 

 from the corresponding chromosomes introduced into the 

 fertilised egg from the two parents, they may be represented 

 as A, j#, C, D... derived from one parent and a, b, c, d... 

 derived from the other. In the maturation divisions A pairs 

 with <2, B with b, and so on, and the members of each pair 

 pass into different germ-cells. There is no reason, however, 

 for supposing that chromosomes of different pairs, but de- 

 rived from the same parent, have any tendency to go to- 

 gether; that is to say, A may equally often be associated 

 with B or with b, and with C or with c. The mature germ- 

 cells may therefore contain A, B, C, D..., A, b, C, /)..., 

 A, b, c, d..., a. By c,D... or any other combinations, so long 

 as no two members of one pair are included in one cell. 

 Now in the segregation of independent Mendelian characters 

 precisely this kind of distribution is found; the characters 

 occur in pairs, which are said to be allelomorphic with each 

 other, and the two members of an allelomorphic pair are 

 never transmitted by the same germ-cell. If two varieties 

 are crossed, differing in three pairs of characters which may 

 be called Af , N, and m, n, 0, the heterozygote will con- 

 tain the "factors" for all six, but its germ-cells will bear, 

 on the average in equal numbers, those for all combinations 

 in which neither Mm, Nn, Oo are included. Or, expressed 



