alone to direct the further activities (Fig.
41). Possibly this may be the case in all eggs, but it is not sure. It
is a matter of some little interest to have this settled, for if it
should prove true, then it would evidently follow that the machinery for
cell division, in the case of sexual reproduction, is derived from the
father, although the bulk of the cell comes from the mother, while the
chromosomes come from both parents.

In the cases where the process has been most carefully studied, the
further changes are as follows: The head of the spermatozoon, after
entrance into the egg, lies dormant until the egg has thrown off its
polar cells, and thus gotten rid of part of its chromosomes. Close to it
lies its centrosomes (Fig. 35, _ce_), and there is thus formed what is
known as the _male pronucleus_ (Fig. 35-40, _mn_). The remains of the
egg nucleus, after having discharged the polar cells, form the _female
nucleus_ (Fig. 40, _fn_). The chromatin material, in both the male and
female pronucleus, soon breaks up into a network in which it is no
longer possible to see that each contains two chromosomes (Fig. 41). Now
the centrosome, which is beside the male pronucleus, shows signs of
activity. It becomes surrounded by prominent rays to form an aster (Fig.
41, _ce_), and then it begins to move toward the female pronucleus,
apparently dragging the male pronucleus after it. In this way the
centrosome approaches the female pronucleus, and thus finally the two
nucleii are brought into close proximity. Meantime the chromatin
material in each has once more broken up into short threads or
chromosomes, and once more we find that each of the nucleii contains two
of these bodies (Fig. 42). In the subsequent figures the chromosomes of
the male nucleus are lightly shaded, while those of the female are black
in order to distinguish them. As these two nucleii finally come together
their membranes disappear, and the chromatic material comes to lie
freely in the egg, the male and female chromosomes, side by side, but
distinct forming the _segmentation nucleus_. The egg plainly now
contains once more the number of chromosomes normal for the cells of the
animal, but half of them have been derived from each parent. It is very
suggestive to find further that the chromosomes in this _fertilized egg_
do not fuse with each other, but remain quite distinct, so that it can
be seen that the new nucleus contains chromosomes derived from each
par