264 RECENT CYTOLOGY 
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 
