ilence of a tropical forest is, after
all, due only to the dullness of our hearing, and could our ears catch
the murmur of these tiny maelstroms, as they whirl in innumerable
myriads of living cells, which constitute each tree, we should be
stunned as with a roar of a great city."

One step higher in the scale of life than the monera is the vegetable or
animal cell, which arose out of the monera by the important process of
segregation in their homogeneous viscid bodies, the differentiation of
an inner kernel from the surrounding plasma. By this means the great
progress from a simple cytod (without kernel) into a real cell (with
kernel) was accomplished. Some of these cells at an early stage encased
themselves by secreting a hardened membrane; they formed the first
vegetable cells, while others remaining naked developed into the first
aggregate of animal cells. The vegetable cell has usually two concentric
coverings--cell-wall and primordial utricle. In animal cells the former
is wanting, the membrane representing the utricle. As a general fact,
also, animal cells are smaller than vegetable cells. Their size[6]
varies greatly, but are generally invisible to the naked eye, ranging
from 1/500 to 1/10000 of an inch in diameter. About four thousand of the
smallest would be required to cover the dot put over the letter i in
writing. The shape of cells varies greatly; the normal form, though, is
spheroidal as in the cells of fat, but they often become[7]
many-sided--sometimes flattened as in the cuticle, and sometimes
elongated into a simple filament as in fibrous tissue or muscular fibre.

The cell, therefore, is extremely interesting, since all animal and
vegetable structure is but the multiplication of the cell as a unit, and
the whole life of the plant or animal is that of the cells which compose
them, and in them or by them all its vital processes are carried on. It
may sound paradoxical to speak of an animal or plant being composed of
millions of cells; but beyond the momentary shock of the paradox no harm
is done.

The cell, then, can be regarded as the basis of our physiological idea
of the elementary organism; but in the animal as well as in the plant,
neither cell-wall nor nucleus is an essential constituent of the cell,
inasmuch as bodies which are unquestionably the equivalents of
cells--true morphological units--may be mere masses of protoplasm,
devoid alike of cell-wall or nucleus. For the whole living world, t