re result from that union two kinds of
cells--namely, immature germ-cells and body-cells; and both these kinds
of cells contain a double system of factors, because of course they have
received a single entire system from each parent. This is the second of
the fundamental principles of Mendelism: that the factors are single in
the mature germ-cell, but in duplicate in the body-cell (and also in the
immature germ-cell).

In every cell with a double system of factors, there are necessarily
present two representatives from each set of allelomorphs, but these may
or may not be alike--or in technical language the individual may be
homozygous, or heterozygous, as regards the given set of alternative
factors. Looking at it from another angle, there is a single visible
character in the plant or animal, but it is produced by a double factor,
in the germ-plasm.

When the immature germ-cell, with its double system of factors, matures,
it throws out half the factors, retaining only a single system: and the
allelomorphic factors which then segregate into different cells are, as
has been said above, ordinarily uninfluenced by their stay together.

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 car