hall give rise to a new individual until it
has united with another cell of quite a different sort and commonly
derived from a different individual called the male. Why the egg cell is
unable to develop without such union with male cell does not concern us
here, but its purpose will be evident as the description proceeds. The
egg cell as it comes from the ovary of the female individual is,
however, not yet ready for union with the male cell, but must first go
through a series of somewhat remarkable changes constituting what is
called _maturation_ of the egg. This phenomenon has such an intimate
relation to all problems connected with the cell, that it must be
described somewhat in detail. There are considerable differences in the
details of the process as it occurs in various animals, but they all
agree in the fundamental points. The following is a general description
of the process derived from the study of a large variety of animals and
plants.

[Illustration Fig. 34.--This and the following figures
represent the process of fertilization of an egg. In all figures _cr_ is
the chromosomes; _cs_ represents the cell substance (omitted in the
following figures); _mc_ is the male reproductive cell lying in contact
with the egg; _mn_ is the male nucleus after entering the egg.]

[Illustration: FIG. 35.--The egg centrosome has divided, and
the male cell with its centrosome has entered the egg.]

In the cells of the body of the animal to which this description applies
there are four chromosomes This is true of all the cells of the animal
except the sexual cells. The eggs arise from the other cells of the
body, but during their growth the chromatin splits in such a way that
the egg contains double the number of chromosomes, i.e., eight (Fig.
34). If this egg should now unite with the other reproductive cell from
the male, the resulting fertilized egg would plainly contain a number of
chromosomes larger than that normal for this species of animal. As a
result the next generation would have a larger number of chromosomes in
each cell than the last generation, since the division of the egg in
development is like that already described and always results in
producing new cells with the same number of chromosomes as the starting
cell. Hence, if the number of chromosomes in the next generation is to
be kept equal to that in the last generation, this egg cell must get rid
of a part of its chromatin material. This is done by a proces