These networks generally perform (or undergo) a most
complex series of changes every time a cell spontaneously divides. In
certain cases, however, it appears that the nucleus divides into two
in a more simple fashion, the rest of the cell contents subsequently
dividing--each half enclosing one part of the previously divided
nucleus. It is by a continued process of cell division that the
complex structures of the most complex organisms is brought about.

The division of a cell, or particle of protoplasm, is indeed a
necessary consequence of its complete nutrition.

For new material can only be absorbed by its surface. But as the cell
grows, the proportion borne by its surface to its mass, continually
decreases; therefore this surface must soon be too small to take in
nourishment enough, and the particle, or cell, must therefore either
die or divide. By dividing, its parts can continue the nutritive
process till their surface, in turn, becomes insufficient, when they
must divide again, and so on. Thus the term "feeding" has two senses.
"To feed a horse," ordinarily means to give it a certain quantity of
hay, oats or what not; and such indeed is one kind of feeding. But
obviously, if the nourishment so taken could not get from the stomach
and intestines into the ultimate particles and cells of the horse's
body, the horse could not be nourished, and still less could it grow.
It is this latter process, called assimilation, which is the real and
essential process of feeding, to which the process ordinarily so
called is but introductory.

Protoplasm has also the power of forming and ejecting from its own
substance, other substances which it has made, but which are of a
different nature to its own. This function, as before said, is termed
secretion; and we know the liver secretes bile, and that the cow's
udder secretes milk.

Here again we have an external and an internal process. The milk is
drawn forth from a receptacle, the udder, into which it finds its way,
and so, in a superficial sense, it may be called an organ of
secretion. Nevertheless the true internal secretion takes place in
the innermost substance of the cells or particles of protoplasm, of
the milk-land, which particles really form that liquid.

But every living creature consists at first entirely of a particle of
protoplasm. Therefore every other kind of substance which may be found
in every kind of plant or animal, must have been formed through it,
and be