mposed of cells,
which are formed and grow according to the same laws wherever they are
found, whose formation therefore is everywhere due to the same forces.
If we find that certain of these cells--all of which we know to be
essentially identical one with another--have the power when separated
from the others of growing and developing into new organisms, we can
infer that not only such cells but also all other cells have this
assimilatory power. The ova of animals, the spores of plants, the
isolated cells of lower organisms in general, all show the power of
separate assimilation and development. "We must therefore, in general,
ascribe to the cell an individual life, that is to say, the combination
of the molecules in the single cell does suffice to produce the force
whereby the cell is enabled to draw to itself new molecules. The ground
of nutrition and growth lies not in the organism as a whole, but in the
separate elementary parts, the cells. The fact that it is not every cell
that can continue to grow when separated from the organism is not in
itself an objection to this theory, any more than it is an objection to
the individual life of a bee that it cannot continue to exist apart from
the swarm. The activation of the forces existing within the cell depends
on conditions which the cell encounters only in connection with the
whole" (pp. 228-9).

Schwann's next step is to discover what are the essential forces active
in the cell, and here he enters the realm of hypothesis. He finds they
can be reduced to two--an attractive force and a metabolic force. The
attractive force is seen in the process of cell-formation, where first
of all the nucleolus is formed by a concentration and precipitation of
substances found free in the cytoblastem, and in the same way the
nucleus and later the cell are laid down as concentric precipitates from
the cytoblastem. Cell-formation also involves the second or metabolic
force, by means of which the cell alters the chemical composition of the
medium surrounding it so as to prepare it for assimilation. Schwann's
attractive force brings about the actual taking up of the prepared
substance; his metabolic force is the cause of the digestion of food
substances, and is nearly identical with enzyme action. With what
inorganic process, he now asks (p. 239), can the process of
cell-formation be most nearly compared, and the answer obviously is,
with the process of crystallisation. Cells are, it is