,
and each half becomes an Infusorian in all appearance identical with the
original cell. Has the parent cell then died? It may rather be said to
survive, in two parts. Each of these daughter cells will in turn go
through the same process of reproduction by simple fission, and the
process will be continued in their descendants. The Infusorian can be
called potentially immortal, because of this method of reproduction.

The immortality, as Weismann pointed out, is not of the kind attributed
by the Greeks to their gods, who could not die because no wound could
destroy them. On the contrary, the Infusorian is extremely fragile, and
is dying by millions at every instant; but if circumstances are
favorable, it _can_ live on; it is not inevitably doomed to die sooner
or later, as is Man. "It dies from accident often, from old age never."

Now the single-celled Infusorian is in many respects comparable with the
single-celled germ of the higher animals. The analogy has often been
carried too far; yet it remains indisputable that the germ-cells of men
reproduce in the same way--by simple fission--as the Infusorian and
other one-celled animals and plants, and that they are organized on much
the same plan. Given favorable circumstances, the germ-cell should be
expected to be equally immortal. Does it ever find these favorable
circumstances?

The investigations of microscopists indicate that it does--that
evolution has provided it with these favorable circumstances, in the
bodies of the higher animals. Let us recall in outline the early history
of the fertilized germ-cell, the _zygote_ formed by the union of ovum
and spermatozooen. These two unite to form a single cell, which is
essentially the same, physiologically, as other germ-cells. It divides
in two similar cells; these each divide; the resulting cells again
divide, and so the process continues, until the whole body--a fully
developed man,--has been produced by division and redivision of the one
zygote.

But the germ-cell is obviously different from most of the cells that
make up the finished product, the body. The latter are highly
differentiated and specialized for different functions--blood cells,
nerve cells, bone cells, muscle cells, and so on, each a single cell but
each adapted to do a certain work, for which the original,
undifferentiated germ-cell was wholly unfit. It is evident that
differentiation began to take place at some point in the series of
divisions, th