they were regarded as accidental or
secondary, and were not thought of as forming any serious objection to
the conception of protoplasm as a definite chemical compound. But modern
opticians improved their microscopes, and microscopists greatly improved
their methods. With the new microscopes and new methods there began to
appear, about twenty years ago, new revelations in regard to this
protoplasm. Its lack of homogeneity became more evident, until there has
finally been disclosed to us the significant fact that protoplasm is to
be regarded as a substance not only of chemical but also of high
mechanical complexity. The idea of this material as a simple homogeneous
compound or as a mixture of such compounds is absolutely fallacious.
Protoplasm is to-day known to be made up of parts harmoniously adapted
to each other in such a way as to form an extraordinarily intricate
machine; and the microscopist of to-day recognizes clearly that the
activities of this material must be regarded as the result of the
machinery which makes up protoplasm rather than as the simple result of
its chemical composition. Protoplasm is a machine and not a chemical
compound.

[Illustration: FIG. 23.--A cell as it appears to the modern microscope.
_a_, protoplasmic reticulum; _b_, liquid in its meshes; _c_, nuclear
membrane; _d_, nuclear reticulum; _e_, chromatin reticulum; _f_,
nucleolus; _g_, centrosome; _h_, centrosphere; _i_, vacuole; _j_, inert
bodies.]

==Structure of Protoplasm==.--The structure of protoplasm is not yet
thoroughly understood by scientists, but a few general facts are known
beyond question. It is thought, in the first place, that it consists of
two quite different substances. There is a somewhat solid material
permeating it, usually, regarded as having a reticulate structure. It is
variously described, sometimes as a reticulate network, sometimes as a
mass of threads or fibres, and sometimes as a mass of foam (Fig. 23,
_a_). It is extremely delicate and only visible under special conditions
and with the best of microscopes. Only under peculiar conditions can it
be seen in protoplasm while alive. There is no question, however, that
all protoplasm is permeated when alive by a minute delicate mass of
material, which may take the form of threads or fibres or may assume
other forms. Within the meshes of this thread or reticulum there is
found a liquid, perfectly clear and transparent, to whose presence the
liquid character of t