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 chromosomes form the part of the cell which contain the hereditary traits handed down from parent to child.  This follows from the fact that the chromosomes are the only part of the cell which, in the fertilized egg, is derived from both parents.  Now the offspring can certainly inherit from each parent, and hence the hereditary traits must be associated with some part of the cell which is derived from both.  But the egg substance is derived from the mother alone; the centrosome, at least in some cases and perhaps in all, is derived only from the father, while the chromosomes are derived from both parents.  Hence it follows that the hereditary traits must be particularly associated with the chromosomes.

With this understanding we can, at least, in part understand the purpose of fertilization.  As we shall see later, it is very necessary in the building of the living machine for each individual to inherit characters from more than one individual.  This is necessary to produce the numerous variations which contribute to the construction of the machine.  For this purpose there has been developed the process of sexual union of reproductive cells, which introduces into the offspring chromatic material from two parents.  But if the two reproductive cells should unite at once the number of chromosomes would be doubled in each generation, and hence be constantly increasing.  To prevent this the polar cells are cast out, which reduces the amount of chromatic material.  The union of the two pronucleii is plainly to produce a