ty is
confined within the narrow limits of a single cell. Thus, the amoeba
begins its life as a cell split off from its parent. This divides in its
turn, and each half is a complete amoeba. When we come a little higher
than the amoeba, we find organisms which consist of several cells, and a
specialization of function begins to appear. As we ascend in the animal
scale, specialization of structure and of function is found continually
advancing, and the various kinds of cells are grouped together into
colonies or organs.

12. Cells and the Human Organism. If the body be studied in its
development, it is found to originate from a single mass of nucleated
protoplasm, a single cell with a nucleus and nucleolus. From this
original cell, by growth and development, the body, with all its various
tissues, is built up. Many fully formed organs, like the liver, consist
chiefly of cells. Again, the cells are modified to form fibers, such as
tendon, muscle, and nerve. Later on, we shall see the white blood
corpuscles exhibit all the characters of the amoeba (Fig. 2). Even such
dense structures as bone, cartilage, and the teeth are formed from cells.

[Illustration: Fig. 2.--Amoeboid Movement of a Human White Blood
Corpuscle. (Showing various phases of movement.)]

In short, cells may be regarded as the histological units of animal
structures; by the combination, association, and modification of these
the body is built up. Of the real nature of the changes going on within
the living protoplasm, the process of building up lifeless material into
living structures, and the process of breaking down by which waste is
produced, we know absolutely nothing. Could we learn that, perhaps we
should know the secret of life.

13. Kinds of Cells. Cells vary greatly in size, some of the smallest
being only 1/3500 an inch or less in diameter. They also vary greatly in
form, as may be seen in Figs. 3 and 5. The typical cell is usually
_globular_ in form, other shapes being the result of pressure or of
similar modifying influences. The globular, as well as the large, flat
cells, are well shown in a drop of saliva. Then there are the _columnar_
cells, found in various parts of the intestines, in which they are closely
arranged side by side. These cells sometimes have on the free surface
delicate prolongations called cilia. Under the microscope they resemble a
wave, as when the wind blows over a field of grain (Fig. 5). There are
besides cells known