those who have no special knowledge of the mechanism of heredity, the
important role the ovum plays. These investigators removed the
ovaries from an albino guinea-pig, and in their place substituted the
ovaries of a black guinea-pig. "From numerous experiments it may be
emphatically stated that normal albinos mated together produce only
albinos." But in this experiment the result was otherwise, for the
albino into which the ovaries of a black guinea-pig were grafted
produced only black offspring. The color-coat of her young,
therefore, was not influenced by her own white hair, but was
determined by the eggs really belonging to the black animal from
which the ovaries were taken; in no other way can the result be
interpreted. It is certain, moreover, that the mode of transmission
of material qualities here exemplified is not exceptional; on the
contrary there is no doubt that the ovum always conveys the sum total
of the qualities the offspring inherits from the mother.

The germinal cells then contain the material basis of inheritance,
and in all probability the substance is located within the nucleus of
the cells. This substance had been seen and studied long before its
relation to the problem of heredity was suspected. Because it takes a
deeper stain than the rest of the nucleus, it stands out prominently
when the cell is treated with certain dyes, and this property
accounts for its name--chromatin. Under such conditions as prevail
just before a cell divides, the chromatic substance is broken up and
reassembled in the form of rods called chromosomes. Curiously enough
the number of rods is uniform for each species of animal, though
different numbers are characteristic of different species; the
characteristic number for man is twenty-four.

Unless some arrangement was made to prevent it, the act of