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 oth