d the _nucleus_. This plays some
part in the nourishment of the cell and also in the formation of new
cells. If it be absent, as is sometimes the case, the cell is short-lived
and unable to reproduce itself. The variety of protoplasm contained in the
nucleus is called the _nucleoplasm_.

[Fig. 4]

Fig. 4--Diagram of a typical cell (after Wilson). 1. Main body. 2. Nucleus.
3. Attraction sphere. 4. Food particles and waste. 5. Cell-wall. 6. Masses
of active material found in certain cells, called plastids.

Surrounding the nucleus is the _main body_ of the cell, sometimes referred
to as the "protoplasm." Since the protoplasm forms all parts of the cell,
this substance is more properly called the _cytoplasm_, or cell plasm.
Surrounding and inclosing the cytoplasm, in many cells, is a thin outer
layer, or membrane, which affords more or less protection to the contents
of the cell. This is usually referred to as the _cell-wall_. A fourth part
of the cell is also described, being called the _attraction sphere_. This
is a small body lying near the nucleus and cooeperating with that body in
the formation of new cells. Food particles, wastes, and other substances
may also be present in the cytoplasm. The parts of a typical cell are
shown in Fig. 4.

*Importance of the Cells.*--The cells must be regarded as the living,
working parts of the body. They are the active agents in all of the
tissues, enabling them to serve their various purposes. Working through
the tissues, they build up the body and carry on its different activities.
They are recognized on this account as _the units of structure and of
function_, and are the "individuals" in the body organization. Among the
most important and interesting of the activities of the cells are those by
which they build up the body, or cause it to grow.

*How the Cells enable the Body to Grow.*--Every cell is able to take new
material into itself and to add this to the protoplasm. This tends to
increase the amount of the protoplasm, thereby causing the cells to
increase in size. A general increase in the size of the cells has the
effect of increasing the size of the entire body, and this is one way by
which they cause it to grow. There is, however, a fixed limit, varying
with different cells, to the size which they attain, and this is quite
low. (The largest cells are scarcely visible to the naked eye.) Any marked
increase in the size of the