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.

We must therefore look upon the organic cell as a little engine with admirably adapted parts.  Within this engine chemical activity is excited.  The fuel supplied to the engine is combined by chemical forces with the oxygen of the air.  The vigour of the oxidation is partly dependent upon temperature, just as it is in any other oxidation process, and is of course dependent upon the presence of fuel to be oxidized, and air to furnish the oxygen.  Unless the fuel is supplied and the air has free access to it, the machine stops, the cell dies.  The energy liberated in this machine is converted into