AND MENDEL'S LAW 247 



nuclear division which ensues differs from all other 

 mitoses in the fact that instead of merely dissevering 

 halves of chromosomes, the actual somatic chromo- 

 somes separate and become distributed equally between 

 the resulting nuclei ; so that in these nuclei, and in the 

 germ nuclei which arise by their division, the number 

 of chromosomes is reduced to half the somatic number. 

 When fertilization takes place the somatic number of 

 chromosomes is restored by the union of nuclei, each 

 of which contains half that number. 



Is it possible to throw any further light upon the 

 meaning of these facts regarding the behaviour of the 

 minute constituent parts of organisms ? 



Let us return to Mendel's experimental discovery, 

 of which an account was given in the last two chapters, 

 and let us consider the case of a cross between parents 

 which differ in respect of two pairs of allelomorphs. 

 Expressing these pairs as A - a and B - b, Mendel 

 showed that the germ-cells of the cross-bred or hetero- 

 zygote bear in equal numbers the combinations AB, Ab, 

 aB, and ab. Now, it seems clear from this behaviour 

 that the allelomorphs must be represented in the cells 

 of the organism by some kind of definite particles, 

 which remain distinct from one another throughout 

 all the cell divisions of the body, since we know that 

 at the formation of the germ-cells these characters are 

 capable of becoming completely segregated. Let us, 

 then, trace the behaviour of the allelomorphs in a 

 diagrammatic way, regarding each as a distinct par- 

 ticle. These particles we may distinguish by certain 

 letters. A and a are the allelomorphs of one pair, 



