THE BEARERS OF THE HERITAGE 297 
be sure of the identity of each individual chromosome, but on the 
other hand, there are some organisms known in which the chromo- 
somes of a single nucleus are not of the same size and form (Fig. 46). 
These latter cases enable us to determine some very significant facts. 
Where such differences of shape and proportion occur they are constant 
in each succeeding division so that similar chromosomes may be iden- 
tified each time. Moreover, in all ordinary mitotic divisions where 
the conditions are accurately known, these chromosomes of different 
types are found to be present as pairs of similar elements; that is, 
there are two of each form or size. 
Pairs of similar chromosomes in the nucleus because one chromo- 
some comes from each parent.—When we recall that the original 
fertilized egg from which the individual develops is really formed by 
the union of two gametes, ovum and spermatozoén, and that each 
off —\y & 
— > .% 
B 
Fic. 46.—A, chromosomes of the mosquito (Culex). (After Stevens.) B, 
chromosomes of the fruit fly (Drosophila). (After Metz.) Both of these forms 
have an unusually small number of chromosomes. (From Guyer.) 
A 
gamete, being a true cell, must carry its own set of chromosomes, the 
significance of the pairs of similar chromosomes becomes evident; one 
of each kind has probably been contributed by each gamete. This 
means that the zygote or fertile ovum contains double the number of 
chromosomes possessed by either gamete, and that, moreover, each 
tissue-cell of the new individual will contain this dual number. For, 
as we have seen, the number of chromosomes is, with possibly a few 
exceptions, constant in the tissue-cells and early germ-cells in suc- 
cessive generations of individuals. For this to be true it is obvious 
that in some way the nuclei of the conjugating gametes have come to 
contain only half the usual number. Technically the tissue-cells are 
said to contain the diploid number of chromosomes, the gametes the 
reduced or haploid number. 
In maturation the number of chromosomes is reduced by one- 
half.—This halving, or as it is known, reduction in the number of 
chromosomes is the essential feature of the process of maturation. It 
