for they are closely
allied to those of fecundation.

The nucleus of an ordinary cell presents itself in the form of a
nearly spherical vesicle. Delicate methods of staining have shown that
the nucleus encloses several round nucleolar corpuscles, and also a
reticulum which is attached to its membrane and spreads through its
whole substance. The liquid part of the nucleus fills the meshes of
this reticular tissue, which stains easily and for this reason is
named _chromatin_. The phenomena of cell division in well-developed
cells with nuclei is termed _mitosis_. Certain lower forms of cells
exist in which the nucleus is not well differentiated. Mitosis begins
in the nucleus (Plate I). Figure 1 represents the cell before division
has commenced. In the protoplasm, by the side of the nucleus, is
formed a small corpuscle (_c_) which is called the _centrosome_. The
nucleus itself is marked _b_. When the cell commences to divide, the
meshes of the network of chromatin contract and the centrosome divides
into two parts (Fig. 2). Shortly afterward the particles of chromatin
concentrate in the form of convoluted rods called _chromosomes_
(Figs. 3 and 4). The number of these varies according to the species
of organism, but remains constant for each animal or vegetable
species. At the same time the two centrosomes separate from each other
on each side of the nucleus. The chromosomes then become shorter and
thicker while the nucleus is completely dissolved in the protoplasm of
the cell, and its membrane disappears (Fig. 4).

Directly afterwards the chromosomes arrange themselves regularly in
line, like soldiers at drill, following one of the larger diameters of
the cell, and forming a barrier between the two centrosomes (Fig. 5).
Each of the chromosomes then divides into two parallel halves of equal
thickness (Fig. 6).

Figures 3 and 4 show that, while these changes are being produced,
each of the two centrosomes is surrounded by stellate rays. Some of
these rays extending in the direction of the chromosomes, become
attached to one of their extremities and draw it toward the
corresponding centrosome (Fig. 7). Thus around each centrosome are
grouped as many chromosomes as the mother cell possessed itself (Fig.
8). Simultaneously, the cell enlarges and its protoplasm commences to
become indented at each end of the diameter previously formed by the
chromosomes. From this moment the nuclear liquid concentrates itself
around each of t