be seen with the naked
eye, while others are so small that they cannot be distinctly seen
with the highest power of the microscope. The living thing or organism
may be composed of a single cell or, in the case of the higher animals
and plants, may be formed of great numbers of cells, those of a
similar character being combined in masses to form organs such as the
liver and brain.

In each cell there is a differentiated area constituting a special
structure, the nucleus, which contains a peculiar material called
"chromatin." The nucleus has chiefly to do with the multiplication of
the cell and contains the factors which determine heredity. The mass
outside of the nucleus is termed "cytoplasm," and this may be
homogeneous in appearance or may contain granules. On the outside
there is a more or less definite cell membrane. It is generally
believed that the cell material has a semi-fluid or gelatinous
consistency and is contained within an intracellular meshwork. It is
an extraordinarily complex mass, whether regarded from a chemical or
physical point of view. (Fig. 1.)

[Illustration: FIG. 1.--DIAGRAM OF CELL. 1. Cell membrane. 2. Cell
substance or cytoplasm. 3. Nucleus. 4. Nuclear membrane.
5. Nucleolus.]

A simple conception of health and disease can be arrived at by the
study of these conditions in a unicellular animal directly under a
microscope, the animal being placed on a glass slide. For this purpose
a small organism called "Amoeba" (Fig. 2), which is commonly present
in freshwater ponds, may be used. This appears as a small mass,
seemingly of gelatinous consistency with a clear outline, the exterior
part homogeneous, the interior granular. The nucleus, which is seen
with difficulty, appears as a small vesicle in the interior. Many
amoebae show also in the interior a small clear space, the contractile
vesicle which alternately contracts and expands, through which action
the movement of the intracellular fluid is facilitated and waste
products removed. The interior granules often change their position,
showing that there is motion within the mass. The amoeba slowly moves
along the surface of the glass by the extension of blunt processes
formed from the clear outer portion which adhere to the surface and
into which the interior granular mass flows. This movement does not
take place by chance, but in definite directions, and may be
influenced. The amoeba will move towards certain substances which may
be placed in