n] MATERIAL BASIS 147 



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

 then be two A'&, two J2'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 different 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, (7..., 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, O..., 

 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 6, 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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