as I
have said, that a Protozooen is an isolated physiological cell, and, like
cells in general, multiplies by means of what Spencer and Haeckel have
aptly called a process of discontinuous growth. That is to say, when a
cell reaches maturity, further growth takes place in the direction of a
severance of its substance--the separated portion thus starting anew as
a distinct physiological unit. But, notwithstanding the complex changes
which have been more recently observed to take place in the nucleus of
some Protozoa prior to their division, the process of multiplication by
division may still be regarded as a process of growth, which differs
from the previous growth of the individual cell in being attended by a
severance of continuity. If we take a suspended drop of gum, and
gradually add to its size by allowing more and more gum to flow into it,
a point will eventually be reached at which the force of gravity will
overcome that of cohesion, and a portion of the drop will fall away from
the remainder. Here we have a rough physical simile, although of course
no true analogy. In virtue of a continuous assimilation of nutriment,
the protoplasm of a cell increases in mass, until it reaches the size at
which the forces of disruption overcome those of cohesion--or, in other
words, the point at which increase of size is no longer compatible with
continuity of substance. Nevertheless, it must not be supposed that the
process is thus merely a physical one. The phenomena which occur even in
the simplest--or so-called "direct"--cell-division, are of themselves
enough to prove that the process is vital, or physiological; and this in
a high degree of specialization. But so, likewise, are all processes of
growth in organic structures; and therefore the simile of the drop of
gum is not to be regarded as a true analogy: it serves only to indicate
the fact that when cell-growth proceeds beyond a certain point
cell-division ensues. The size to which cells may grow before they thus
divide is very variable in different kinds of cells; for while some may
normally attain a length of ten or twelve inches, others divide before
they measure 1/1000 of an inch. This, however, is a matter of detail,
and does not affect the general physiological principles on which we are
at present engaged.

Now, as we have seen, a Protozooen is a single cell; for even although in
some of the higher forms of protozoal life a colony of cells may be
bound together in