Between this common mitosis, or indirect cell-division--which is the
normal cleavage-process in most cells of the higher animals and
plants--and the simple direct division (Figure 1.10) we find every
grade of segmentation; in some circumstances even one kind of division
may be converted into another.

The plastid is also endowed with the functions of movement and
sensation. The single cell can move and creep about, when it has space
for free movement and is not prevented by a hard envelope; it then
thrusts out at its surface processes like fingers, and quickly
withdraws them again, and thus changes its shape (Figure 1.12).
Finally, the young cell is sensitive, or more or less responsive to
stimuli; it makes certain movements on the application of chemical and
mechanical irritation. Hence we can ascribe to the individual cell all
the chief functions which we comprehend under the general heading of
"life"--sensation, movement, nutrition, and reproduction. All these
properties of the multicellular and highly developed animal are also
found in the single animal-cell, at least in its younger stages. There
is no longer any doubt about this, and so we may regard it as a solid
and important base of our physiological conception of the elementary
organism.

Without going any further here into these very interesting phenomena
of the life of the cell, we will pass on to consider the application
of the cell theory to the ovum. Here comparative research yields the
important result that EVERY OVUM IS AT FIRST A SIMPLE CELL. I say this
is very important, because our whole science of embryology now
resolves itself into the problem: "How does the multicellular organism
arise from the unicellular?" Every organic individual is at first a
simple cell, and as such an elementary organism, or a unit of