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.
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