ons were combined
together by Schwann into one general theory. According to the cell
doctrine then formulated, the parts of all animals and plants are either
composed of cells or of material derived from cells. The bark, the wood,
the roots, the leaves of plants are all composed of little vesicles
similar to those already described under the name of cells. In animals
the cellular structure is not so easy to make out; but here too the
muscle, the bone, the nerve, the gland are all made up of similar
vesicles or of material made from them. The cells are of wonderfully
different shapes and widely different sizes, but in general structure
they are alike. These cells, thus found in animals and plants alike,
formed the first connecting link between animals and plants. This
discovery was like that of our supposed supramundane observer when he
first found the human being that brought into connection the widely
different cities in the various parts of the world.

[Illustration: FIG. 7.--A bit of bark showing cellular structure.]

Schwann and his immediate followers, while recognizing that the bodies
of animals and plants were composed of cells, were at a loss to explain
how these cells arose. The belief held at first was that there existed
in the bodies of animals and plants a structureless substance which
formed the basis out of which the cells develop, in somewhat the same
way that crystals arise from a mother liquid. This supposed substance
Schwann called the _cytoblastema_, and he thought it existed between the
cells or sometimes within them. For example, the fluid part of the blood
is the cytoblastema, the blood corpuscles being the cells. From this
structureless fluid the cells were supposed to arise by a process akin
to crystallization. To be sure, the cells grow in a manner very
different from that of a crystal. A crystal always grows by layers being
added upon its outside, while the cells grow by additions within its
body. But this was a minor detail, the essential point being that from a
structureless liquid containing proper materials the organized cell
separated itself.

This idea of the cytoblastema was early thrown into suspicion, and
almost at the time of the announcement of the cell doctrine certain
microscopists made the claim that these cells did not come from any
structureless medium, but by division from other cells like themselves.
This claim, and its demonstration, was of even greater importance than
the