ous activities of the nucleus soon
caused it to be regarded as the important part of the cell, while all
the rest was secondary. The cell was now thought of as a bit of nuclear
matter surrounded by secondary parts. The marvellous activities of the
nucleus, and above all, the fact that the nucleus alone is handed down
from one generation to the next in reproduction, all attested to its
great importance and to the secondary importance of the rest of the
cell.

This was the most extreme position of the cell doctrine. The cell was
the unit of living action, and the higher animal or plant simply a
colony of such units. An animal was simply an association together for
mutual advantage of independent units, just as a city is an association
of independent individuals. The organization of the animals was simply
the result of the combination of many independent units. There was no
activity of the organism as a whole, but only of its independent parts.
Cell life was superior to organized life. Just as, in a city, the city
government is a name given to the combined action of the individuals, so
are the actions of organisms simply the combined action of their
individual cells.

Against such an extreme position there has been in recent years a
decided reaction, and to-day it is becoming more and more evident that
such a position cannot be maintained. In the first place, it is becoming
evident that the cell substance is not to be entirely obliterated by the
importance of the nucleus. That the nucleus is a most important vital
centre is clear enough, but it is equally clear that nucleus and cell
substance must be together to constitute the life substance. The
complicated structure of the cell substance, the decided activity shown
by its fibres in the process of cell division, clearly enough indicate
that it is a part of the cell which can not be neglected in the study of
the life substance. Again the discovery of the centrosome as a distinct
morphological element has still further added to the complexity of the
life substance, and proved that neither nucleus nor cell substance can
be regarded as the cell or as constituting life. It is true that we may
not yet know the source of this centrosome. We do not know whether it is
handed down from generation to generation like the nucleus, or whether
it can be made anew out of the cell substance in the life of an ordinary
cell. But this is not material to its recognition as an organ of
impor