During this time the chromosomes have been changing their position.  Whether this change in position is due to forces within themselves, or whether they are moved around passively by forces residing in the cell substances, or whether, which is the most probable, they are pulled or pushed around by the spindle fibres which are forcing their way into the nucleus, is not positively known; nor is it, for our purposes, of special importance.  At all events, the result is that when the asters have assumed their position at opposite poles of the nucleus the chromosomes are arranged in a plane passing through the middle of the nucleus at equal distances from each aster.  It seems certain that they are pulled or pushed into this position by forces radiating from the centrosomes.  Fig. 30 shows this central arrangement of the chromosomes, forming what is called the equatorial plate.

The next step is the most significant of all.  It consists in the splitting of each chromosome into two equal halves.  The threads do not divide in their middle but split lengthwise, so that there are formed two halves identical in every respect.  In this way are produced twice the original number of chromosomes, but all in pairs.  The period at which this splitting of the chromosomes occurs is not the same in all cells.  It may occur, as described, at about the time the asters have reached the opposite poles of the nucleus, and an equatorial plate is formed.  It is not infrequent, however, for it to occur at a period considerably earlier, so that the chromosomes are already divided when they are brought into the equatorial plate.

At some period or other in the cell division this splitting of the chromosomes takes place.  The significance of the splitting is especially noteworthy.  We shall soon find reason for believing that the chromosomes contain all the hereditary traits which the cell hands down from generation to generation, and indeed that the chromosomes of the egg contain all the traits which the parent hands down to the child.  Now, if this chromatin thread consists of a series of units, each representing certain hereditary characters, then it is plain that the division of the thread by splitting will give rise to a double series of threads, each of which has identical characters.  Should the division occur across the thread the two halves would be unlike, but taking place as it does by a longitudinal splitting each unit in the thread simply divides in half, and thus the resulting half threads each contain the same number of similar units as the other and the same as possessed by the original undivided chromosome.  This sort of splitting thus doubles the number of chromosomes, but produces no differentiation of material.

[Illustration:  FIG. 31.—­Stage showing the two halves of the chromosomes separated from each other.]

[Illustration:  FIG. 32.—­Final stage with two nucleii in which the chromosomes have again assumed the form of a network.  The centrosomes have divided preparatory to the next division, and the cell is beginning to divide.]