But the allelomorphic factors are not the only ones which are segregated into different germ-cells, at the maturation of the cell; for the factors which are not alternative are likewise distributed, more or less independently of each other, so that it is largely a matter of chance whether factors which enter a cross in the same germ-cell, segregate into the same germ-cell or different ones, in the next generation. This is the next fundamental principle of Mendelism, usually comprehended under the term "segregation," although, as has been pointed out, it is really a double process, the segregation of alternative factors being a different thing from the segregation of non-alternative factors. From this fact of segregation, it follows that as many kinds of germ-cells can be formed by an individual, as there are possible combinations of factors, on taking one alternative from each pair of allelomorphs present. In practice, this means that the possible number of different germ-cells is almost infinitely great, as would perhaps be suspected by anyone who has tried to find two living things that are just alike. [Illustration: THE CARRIERS OF HEREDITY FIG. 46.--Many different lines of study have made it seem probable that much, although not all, of the heredity of an animal or plant is carried in the nucleus of the germ-cell and that in this nucleus it is further located in little rods or threads which can be easily stained so as to become visible, and which have the name of chromosomes. In the above illustration four different views of the nucleus of the germ-cell of an earthworm are shown, with the chromosomes in different stages; in section 19 each chromosome is doubled up like a hairpin. Study of the fruit-fly Drosophila has made it seem probable not only that the hypothetical factors of heredity are located in the chromosomes, but that each factor has a perfectly definite location in its chromosome; and T. H. Morgan and his associates have worked out an ingenious method of measuring the distance from either end, at which the factor lies. Photomicrograph after Foot and Strobell.] Such is the essence of Mendelism; and the reader is probably ready to admit that it is not a simple matter, even when reduced to the simplest terms. To sum up, the principal features at the base of the hypothetical structure are these: 1. There exist relatively constant units in the germ-plasm. 2. There are two very distinct relationships which these units may show to each other. Two (or more) unit factors may be alternatives in the mechanism of inheritance, indicating that one is a variation (or loss) of the other; or they may be independent of each other in the mechanism of inheritance. 3. The mature germ-cell contains a single system of independent factors (one representative from each set of alternates). The immature germ-cells, and body-cells, have double systems of independent factors (two from each set of alternatives).