er and the same as possessed by the original
undivided chromosome. This sort of splitting thus doubles the number of
chromosomes, but produces no differentiation of material.

[Illustration: FIG. 31.--Stage showing the two halves of the
chromosomes separated from each other.]

[Illustration: FIG. 32.--Final stage with two nucleii in which
the chromosomes have again assumed the form of a network. The
centrosomes have divided preparatory to the next division, and the cell
is beginning to divide.]

The next step in the cell division consists in the separation of the two
halves of the chromosomes. Each half of each chromosome separates from
its fellow, and moves to the opposite end of the nucleus toward the two
centrosomes (Fig. 31). Whether they are pulled apart or pushed apart by
the spindle fibres is not certain, although it is apparently sure that
these fibres from the centrosomes are engaged in the matter. Certain it
is that some force exerted from the two centrosomes acts upon the
chromosomes, and forces the two halves of each one to opposite ends of
the nucleus, where they now collect and form two _new nucleii_, with
evidently exactly the same number of chromosomes as the original, and
with characters identical to each other and to the original (Fig. 32).

The rest of the cell division now follows rapidly. A partition grows in
through the cell body dividing it into two parts (Fig. 32), the division
passing through the middle of the spindle. In this division, in some
cases at least, the spindle fibres bear a part--a fact which again
points to the importance of the centrosomes and the forces which radiate
from them. Now the chromosomes in each daughter nucleus unite to form a
single thread, or may diffuse through the nucleus to form a network, as
in Fig. 32. They now become surrounded by a membrane, so that the new
nucleus appears exactly like the original one. The spindle fibres
disappear, and the astral fibres may either disappear or remain visible.
The centrosome may apparently in some cases disappear, but more commonly
remains beside the daughter nucleii, or it may move into the nucleus.
Eventually it divides into two, the division commonly occurring at once
(Fig. 32), but sometimes not until the next cell division is about to
begin. Thus the final result shows two cells each with a nucleus and
two centrosomes, and this is exactly the same sort of structure with
which the process began. (_See Frontispiece_.)