as _spindle, stellate, squamous_ or pavement, and
various other names suggested by their shapes. Cells are also described as
to their contents. Thus _fat_ and _pigment_ cells are alluded to in
succeeding sections. Again, they may be described as to their functions or
location or the tissue in which they are found, as _epithelial_ cells,
_blood_ cells (corpuscles, Figs. 2 and 66), _nerve_ cells (Fig. 4), and
_connective-tissue_ cells.

14. Vital Properties of Cells. Each cell has a life of its own. It
manifests its vital properties in that it is born, grows, multiplies,
decays, and at last dies.[3] During its life it assimilates food, works,
rests, and is capable of spontaneous motion and frequently of locomotion.
The cell can secrete and excrete substance, and, in brief, presents nearly
all the phenomena of a human being.

Cells are produced only from cells by a process of self-division,
consisting of a cleavage of the whole cell into parts, each of which
becomes a separate and independent organism. Cells rapidly increase in
size up to a certain definite point which they maintain during adult life.
A most interesting quality of cell life is motion, a beautiful form of
which is found in ciliated epithelium. Cells may move actively and
passively. In the blood the cells are swept along by the current, but the
white corpuscles, seem able to make their way actively through the
tissues, as if guided by some sort of instinct.

[Illustration: Fig. 3.--Various Forms of Cells.

A, columnar cells found lining various parts of the intestines (called
_columnar epithelium_);
B, cells of a fusiform or spindle shape found in the loose tissue under
the skin and in other parts (called _connective-tissue cells_);
C, cell having many processes or projections--such are found in
connective tissue, D, primitive cells composed of protoplasm with
nucleus, and having no cell wall. All are represented about 400 times
their real size.
]

Some cells live a brief life of 12 to 24 hours, as is probably the case
with many of the cells lining the alimentary canal; others may live for
years, as do the cells of cartilage and bone. In fact each cell goes
through the same cycle of changes as the whole organism, though doubtless
in a much shorter time. The work of cells is of the most varied kind, and
embraces the formation of every tissue and product,--solid, liquid, or
gaseous. Thus we shall learn that the cells of the