means of such a differentiation there arises a very complex mass of
cells, with great variety in shape and function.

[Illustration: FIG. 19. A piece of nerve fibre, showing the cell with
its nucleus at _n_.]

It should be noticed further that there are some animals and plants in
which the whole animal is composed of a single cell. These organisms
are usually of extremely minute size, and they comprise most of the
so-called animalculae which are found in water. In such animals the
different parts of the cell are modified to perform different functions.
The different organs appear within the cell, and the cell is more
complex than the typical cell described. Fig. 21 shows such a cell. Such
an animal possesses several organs, but, since it consists of a single
mass of protoplasm and a single nucleus, it is still only a single cell.
In the multicellular organisms the organs of the body are made up of
cells, and the different organs are produced by a differentiation of
cells, but in the unicellular organisms the organs are the results of
the differentiation of the parts of a single cell. In the one case there
is a differentiation of cells, and in the other of the parts of a cell.

[Illustration: FIG. 20.--A muscle fibre. The nucleii are shown at _n_.]

[Illustration: FIG. 21.--A complex cell. It is an entire animal, but
composed of only one cell.]

Such, in brief, is the cell to whose activities it is possible to trace
the fundamental properties of all living things. Cells are endowed with
the properties of irritability, contractibility, assimilation and
reproduction, and it is thus plainly to the study of cells that we must
look for an interpretation of life phenomena. If we can reach an
intelligible understanding of the activities of the cell our problem is
solved, for the activities of the fully formed animal or plant, however
complex, are simply the application of mechanical and chemical
principles among the groups of such cells. But wherein does this
knowledge of cells help us? Are we any nearer to understanding how these
vital processes arise? In answer to this question we may first ask
whether it is possible to determine whether any one part of the cell is
the seat of its activities.

==The Cell Wall.==--The first suggestion which arose was that the cell
wall was the important part of the cell, the others being secondary.
This was not an unnatural conclusion. The cell wall is the most
persistent part of the ce