normally, neither boys, girls, men, nor women, can have bodies or
brains worth talking about. It is a process, without which not a step
can be taken, or muscle moved, or food digested, or nutriment
assimilated, or any function, physical or mental, performed. By its
aid, growth and development are carried on. Youth, maturity, and old
age result from changes in its character. It is alike the support and
the guide of health convalescence, and disease. It is the means by
which, in the human system, force is developed, and growth and decay
rendered possible. The process, in itself, is one of the simplest. It
is merely the replacing of one microscopic cell by another; and yet
upon this simple process hang the issues of life and death, of thought
and power.

Carpenter, in his physiology, reports the discovery, which we owe to
German investigation, "that the whole structure originates in a single
cell; that this cell gives birth to others, analogous to itself, and
these again to many future generations; and that all the varied
tissues of the animal body are developed from cells."[9] A more recent
writer adds, "In the higher animals and plants, we are presented with
structures which may be regarded as essentially aggregates of cells;
and there is now a physiological division of labor, some of the cells
being concerned with the nutriment of the organism, whilst others are
set apart, and dedicated to the function of reproduction. Every cell
in such an aggregate leads a life, which, in a certain limited sense,
may be said to be independent; and each discharges its own function in
the general economy. Each cell has a period of development, growth,
and active life, and each ultimately perishes; the life of the
organism not only not depending upon the life of its elemental
factors, but actually being kept up by their constant destruction and
as constant renewal."[10] Growth, health, and disease are cellular