ed materials); the nucleus with its contained chromatin
and nuclear sap. (After Dahlgren.)]

Throughout the greater part of the last century, while students of
evolution and of heredity were engaged in what I may call the more general,
or, shall I say, the _grosser_ aspects of the subject, there existed
another group of students who were engaged in working out the minute
structure of the material basis of the living organism. They found that
organs such as the brain, the heart, the liver, the lungs, the kidneys,
etc., are not themselves the units of structure, but that all these organs
can be reduced to a simpler unit that repeats itself a thousand-fold in
every organ. We call this unit a cell (fig. 45).

The egg is a cell, and the spermatozoon is a cell. The act of fertilization
is the union of two cells (fig. 47, upper figure). Simple as the process of
fertilization appears to us today, its discovery swept aside a vast amount
of mystical speculation concerning the role of the male and of the female
in the act of procreation.

Within the cell a new microcosm was revealed. Every cell was found to
contain a spherical body called the nucleus (fig. 46a). Within the nucleus
is a network of fibres, a sap fills the interstices of the network. The
network resolves itself into a definite number of threads at each division
of the cell (fig. 46 b-e). These threads we call chromosomes. Each species
of animals and plants possesses a characteristic number of these threads
which have a definite size and sometimes a specific shape and even
characteristic granules at different levels. Beyond this point our
strongest microscopes fail to penetrate. Observation has reached, for the
time being, its limit.

[Illustration: FIG. 46. A series of cells in process of cell division. The
chromosomes are the black threads and rods. (After Dahlgren.)]

The story is taken up at this point by a new set of students who have
worked in an entirely different field. Certain observations and experiments
that we have not time to consider now, led a number of biologists to
conclude that the chromosomes are the bearers of the hereditary units. If
so, there should be many such units carried by _each_ chromosome, for the
number of chromosomes is limited while the number of independently
inherited characters is large. In Drosophila it has been demonstrated not
only that there are exactly as many groups of characters that are inherited
together as there are pa