ried 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).

4. The double system arises simply from the union of two single systems
(i. e., two germ-cells), without union or even contamination of the
factors involved.

In the formation of a single system (mature germ-cells) from a double
(immature germ-cells), pairs of alternates separate, passing into
different germ-cells. Factors not alternates may or may not
separate--the distribution is largely a matter of chance.

Such are the fundamental principles of Mendelism; but on them was early
grafted a theoretical structure due mainly to the German zooelogist,
August Weismann. To understand his part in the story, we must advert to
that much mooted and too often misunderstood problem furnished by the
chromosomes. (See Fig. 46.) These little rods of easily stained
material, which are found in every ce