ll. It was the part first discovered by the
microscope and is the part which remains after the other parts are gone.
Indeed, in many of the so-called cells the cell wall is all that is
seen, the cell contents having disappeared (Fig. 14). It was not
strange, then, that this should at first have been looked upon as the
primary part. The idea was that the cell wall in some way changed the
chemical character of the substances in contact with its two sides, and
thus gave rise to vital activities which, as we have seen, are
fundamentally chemical. Thus the cell wall was regarded as the most
essential part of the cell, since it controlled its activities. This the
belief of Schwann, although he also regarded the other parts of the
cell as of importance.

[Illustration: FIG. 22.--An amoeba. A single cell without cell wall. _n_
is the nucleus; _f_, a bit of food which the cell has absorbed.]

This conception, however, was quite temporary. It was much as if our
hypothetical supramundane observer looked upon the clothes of his newly
discovered human being as forming the essential part of his nature. It
was soon evident that this position could not be maintained. It was
found that many bits of living matter were entirely destitute of cell
wall. This is especially true of animal cells. While among plants the
cell wall is almost always well developed, it is very common for animal
cells to be entirely lacking in this external covering--as, for example,
the white blood-cells. Fig. 22 shows an amoeba, a cell with very active
powers of motion and assimilation, but with no cell wall. Moreover,
young cells are always more active than older ones, and they commonly
possess either no cell wall or a very slight one, this being deposited
as the cell becomes older and remaining long after it is dead. Such
facts soon disproved the notion that the cell wall is a vital part of
the cell, and a new conception took its place which was to have a more
profound influence upon the study of living things than any discovery
hitherto made. This was the formulation of the doctrine of the nature
of _protoplasm_.

Protoplasm.--(a) _Discovery_. As it became evident that the cell wall is
a somewhat inactive part of the cell, more attention was put on the cell
contents. For twenty years after the formulation of the cell doctrine
both the cell substance and the nucleus had been looked upon as
essential to its activities. This was more especially true of the
nucl