ent (Fig. 42). Nor does there appear to be, in the future history of
this egg, any actual fusion of the chromatic material, the male and
female chromosomes perhaps always remaining distinct.

[Illustration: FIG. 41.--The chromosomes in the male and female
pronucleii have resolved into a network. The male centrosome begins to
show signs of activity.]

[Illustration: FIG. 42.--The centrosome has divided, and the two
pronucleii have been brought together. The network in each nucleus has
again resolved itself into two chromosomes which are now brought
together near the centre of the egg but do not fuse; _mcr_, represents
the chromosomes from the male nucleus; _fcr_, the chromosomes from the
female nucleus.]

[Illustration: FIG. 43.--An equatorial plate is formed and each
chromosome has split into two halves by longitudinal division.]

[Illustration: FIG. 44.--The halves of the chromosomes have separated to
form two nucleii, each with male and female chromosomes. The egg has
divided into two cells.]

While this mixture of chromosomes has been taking place the centrosome
has divided into two parts, each of which becomes surrounded by an aster
and travels to opposite ends of the nucleus (Fig. 42). There now follows
a division of the nucleus exactly similar to that which occurs in the
normal cell division already described in Figs. 28-34. Each of the
chromosomes splits lengthwise (Fig. 43), and one half of each then
travels toward each centrosome to form a new nucleus (Fig. 44). Since
each of the four chromosomes thus splits, it follows that each of the
two daughter nucleii will, of course, contain four chromosomes; two of
which have been derived from the male and two from the female parent.
From now the divisions of the egg follow rapidly by the normal process
of cell division until from this one egg cell there are eventually
derived hundreds of thousands of cells which are gradually moulded into
the adult. All of these cells will, of course, contain four chromosomes;
and, what is more important, half of the chromosomes will have been
derived directly from the male and half from the female parent. Even
into adult life, therefore, the cells of the animal probably contain
chromatin derived by direct descent from each of its parents.

==The Significance of Fertilization.==--From this process of fertilization
a number of conclusions, highly important for our purpose, can be drawn.
In the first place, it is evident that the