The small particles of protoplasm which constitute “cells” are far indeed from being structureless.  Besides the nucleus already mentioned there is a delicate network of threads of a substance called chromatin within it, and another network permeating the fluid of the cell substance, which invest the nucleus often with further complications.  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.