expand with heat, as they may
also do from imbibing moisture; but living protoplasm has an
apparently spontaneous power of contraction and expansion under
certain external conditions which do not occasion such movements in
inorganic matter.

[Illustration: FIG. 3. CELL FROM A SALAMANDER. _n_, nucleus; _n'_,
nucleolus embedded in the network of chromatin threads; _k_, network
of the cell external to the nucleus; _a_, attraction-sphere or
archoplasm containing minute bodies called centrosomes; _cl_, membrane
enclosing the cell externally, _nl_, membrane surrounding the nucleus;
_c_, centrosomes.]

Under favoring conditions, protoplasm has a power of performing
chemical changes, which result in producing heat far more gently and
continuously than it is produced by the combustion of inorganic
bodies. Thus it is that the heat is produced which makes its presence
evident to us in what we call "warm-blooded animals," the most
warm-blooded of all being birds.

Protoplasm has also the wonderful power of transforming certain
adjacent substances into material like itself--into its own
substance--and so, in a sense, creating a new material. Thus it is
that organisms have the power to nourish themselves and grow. An
animal would vainly swallow the most nourishing food if the ultimate,
protoplasmic particles of its body had not this power of
"transforming" suitable substances brought near them in ways to be
hereinafter noticed.

Without that, no organism could ever "grow." The growth of organisms
is utterly different from the increase in size of inorganic bodies.
Crystals, as we have seen, grow merely by external increment; but
organisms grow by an increment which takes place in the very innermost
substance of the tissues which compose their bodies, and the innermost
substance of the cells which compose such tissues; this peculiar form
of growth is termed _intussusception_.

Protoplasm, after thus augmenting its mass, has a further power of
spontaneous division, whereby the mass of the entire organism whereof
such protoplasm forms a part, is augmented and so growth is brought
about.

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.