Viewed as a whole, we may make the following general summary of this
process. The essential object of this complicated phenomena of
_karyokinesis_ is to divide the chromatin into equivalent halves, so
that the cells resulting from the cell division shall contain an exactly
equivalent chromatin content. For this purpose the chromatic elements
collect into threads and split lengthwise. The centrosome, with its
fibres, brings about the separation of these two halves. Plainly, we
must conclude that the chromatin material is something of extraordinary
importance to the cell, and the centrosome is a bit of machinery for
controlling its division and thus regulating cell division.

==Fertilization of the Egg.==--This description of cell division will
certainly give some idea of the complexity of cell life, but a more
marvelous series of changes still takes place during the time when the
egg is preparing for development. Inasmuch as this process still further
illustrates the nature of the cell, and has further a most intimate
bearing upon the fundamental problem of heredity, it will be necessary
for us to consider it here briefly.

The sexual reproduction of the many-celled animals is always essentially
alike. A single one of the body cells is set apart to start the next
generation, and this cell, after separating from the body of the animal
or plant which produced it, begins to divide, as already shown in Fig.
8, and the many cells which arise from it eventually form the new
individual This reproductive cell is the egg. But before its division
can begin there occurs in all cases of sexual reproduction a process
called fertilization, the essential feature of which is the union of
this cell with another commonly from a different individual. While the
phenomenon is subject to considerable difference in details, it is
essentially as follows:

[Illustration: FIG. 33--An egg showing the cell substance and
the nucleus, the latter containing chromosomes in large number and a
nucleolus]

The female reproductive cell is called the egg, and it is this cell
which divides to form the next generation. Such a cell is shown in Fig.
33. Like other cells it has a cell wall, a cell substance with its linin
and fluid portions, a nucleus surrounded by a membrane and containing a
reticulum, a nucleolus and chromatic material, and lastly, a centrosome.
Now such an egg is a complete cell, but it is not able to begin the
process of division which s