II] MATERIAL BASIS 147 



are represented by letters, A, B, C, D..., there will 

 then be two A'b, two ^'s, etc. in the nucleus. The 

 actual processes in the reduction division are some- 

 what complex, but briefly they consist in a pairing 

 together of the chromosomes, followed by a division 

 of the nucleus in which the two members of each 

 pair are separated into diflerent daughter-nuclei, 

 so that the daughter-nuclei each contain half the 

 full number. When the chromosomes differ among 

 themselves, it is seen that two similar ones always 

 pair together, i.e. A with A, B with B, etc. Thus 

 the daughter-nuclei each contain the whole series 

 A, B, C. . ., but have only one of each, instead of two. 

 If then it is imagined that each chromosome is 

 the bearer of the determinant (or 'factor') for a 

 Mendelian character, we may regard one individual 

 as having a double series of chromosomes A, B, C. . ., 

 etc., and another as bearing the allelomorphic cha^ 

 racters a, b, c..., etc. When these individuals are 

 mated, the heterozygote will bear both series, 

 A and a, B and b, etc. In the formation of the 

 germ-cells, A will segregate from a, B from b in 

 exactly the way required by Mendelian theory. But 

 there is no reason to suppose the series A, B, C... 

 should all go into one germ-cell, and a, b,c... into the 

 other ; A may go into the first daughter-nucleus and 

 a into the second, but b may go with A into the first, 

 and B into the second. So in crossing races differing 



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