[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 coöperating 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.