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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. Now the centrosome, which is beside the male pronucleus, shows signs of activity.
This process of the formation of the polar cells is thus simply a device for getting rid of some of the chromatin material in the egg cell, so that it may unite with a second cell without doubling the normal number of chromosomes. Completion of the process of extrusion of the chromatic material; fn shows the two chromosomes retained in the egg forming the female pronucleus.
In this way the centrosome approaches the female pronucleus, and thus finally the two nucleii are brought into close proximity. 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.
This entrance of the male cell occurs either before the formation of the polar cells of the egg or afterward. If, however, it takes place before, the male pronucleus simply remains dormant in the egg while the polar cells are being protruded, and not until after that process is concluded does it begin again to show signs of activity which result in the cell union.
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