=Pairs of Similar Chromosomes in the Nucleus Because One Chromosome 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 spermatozoon, and that each 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 successive 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. [Illustration: FIG. 8 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.] =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 is accomplished by a modification in the mitotic division in which instead of each chromosome splitting lengthwise, as in ordinary mitosis, the chromosomes unite in pairs (Fig. 9_b_, p. 42), a process known technically as _synapsis_, and then apparently one member of each pair passes entire into one new daughter cell, the other member going to the other daughter cell (Fig. 9_c_, p. 42). In the pairing preliminary to this _reduction division_, leaving out of account certain special cases to be considered later, according to the best evidence at our command the union always takes place between two chromosomes which match each other in size and appearance. Since one of these is believed to be of maternal and the other of paternal origin, the ensuing division separates corresponding mates and insures that each gamete gets one of each kind of chromosome although it appears to be a matter of mere chance whether or not a given cell gets the paternal or the maternal representative of that kind. [Illustration: FIG. 9