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 parent (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.]