tilization of the egg_. Every animal and plant begins its life as a
simple cell, and the growth of the cell into the adult is simply the
division of the original cell into parts accompanied by a
differentiation of the parts. The fundamental phenomena of growth and
reproduction is thus cell division, and if we can comprehend this
process in these simple cells we shall certainly have taken a great step
toward the explanation of the mechanics of life. During the last ten
years this cell division has been most thoroughly studied, and we have a
pretty good knowledge of it so far as its microscopical features are
concerned. The following description will outline the general facts of
such cell division, and will apply with considerable accuracy to all
cases of cell division, although the details may differ not a little.

[Illustration: FIG. 27.--This and the following figures show
stages in cell division. Fig. 27 shows the resting stage with the
chromatin, _cr_, in the form of a network within the nuclear membrane
and the centrosome, _ce_, already divided into two.]

[Illustration: FIG. 28.--The chromatin is broken into threads or
chromosomes, _cr._ The centrosomes show radiating fibres.]

==Cell Division or Karyokinesis.==--We will begin with a cell in what is
called the resting stage, shown at Fig. 23. Such a cell has a nucleus,
with its chromatin, its membrane, and linin, as already described.
Outside the nucleus is the centrosome, or, more commonly, two of them
lying close together. If there is only one it soon divides into two, and
if it has already two, this is because a single centrosome which the
cell originally possessed has already divided into two, as we shall
presently see. This cell, in short, is precisely like the typical cell
which we have described, except in the possession of two centrosomes.
The first indication of the cell division is shown by the chromatin
fibres. During the resting stage this chromatin material may have the
form of a thread, or may form a network of fibres (see Fig. 27). But
whatever be its form during the resting stage, it assumes the form of a
thread as the cell prepares for division. Almost at once this thread
breaks into a number of pieces known as _chromosomes_ (Fig. 28). It is
an extremely important fact that the number of these chromosomes in the
ordinary cells of any animal or plant is always the same. In other
words, in all the cells of the body of animal or plant the chromatin