of the nucleus,
even though they have only a small portion of the cell substance, feed,
assimilate, and grow; in other words, they carry on not only the
destructive but also the constructive changes. Plainly, this means that
the nucleus controls the constructive processes, although it does not
necessarily mean that the cell substance has no share in these
constructive processes. Without the nucleus the cell is unable to
perform those processes, while it is able to carry on the destructive
processes readily enough. The nucleus controls, though it may not
entirely carry on, the constructive metabolism.

It is equally clear that the _cell substance_ is the seat of most of the
destructive processes which constitute vital action. The cell substance
is irritable, and is endowed with the power of contractility. Cell
fragments without nucleii are sensitive enough, and can move around as
readily as normal cells. Moreover, the various fibres which surround the
centrosomes in cell division and whose contractions and expansions, as
we have seen, pull the chromosomes apart in cell division, are parts of
the cell substance. All of these are the results of destructive
metabolism, and we must, therefore, conclude that destructive processes
are seated in the cell substance.

The _centrosome_ is too problematical as yet for much comment. It
appears to be a piece of the machinery for bringing about cell division,
but beyond this it is not safe to make any statements.

In brief, then, the cell body is a machine for carrying on destructive
chemical changes, and liberating from the compounds thus broken to
pieces their inclosed energy, which is at once converted into motion or
heat or some other form of active energy. This chemical destruction is,
however, possible only after the chemical compounds have become a part
of the cell. The cell, therefore, possesses a nucleus which has the
power of enabling it to assimilate its food--that is, to convert it into
its own substance. The nucleus further contains a marvellous
material--chromatin--which in someway exercises a controlling influence
in its life and is handed down from one generation to another by
continuous descent. Lastly, the cell has the centrosome, which brings
about cell division in such a manner that this chromatin material is
divided equally among the subsequent descendants, and thus insures that
the daughter cells shall all be equivalent to each other and to the
mother cell.