THE GERM-CELLS 131

need endowing with properties unlike those of all
other organic agents." " Thus, there is no warrant
for the assumption that sperm-cells and germ-cells
possess powers fundamentally unlike those of other
cells."

To this it must be answered: (l) though sperm-
cells and egg-cells, being living units, cannot be
"fundamentally unlike" other living units, such as
ordinary body-cells, yet they may be very unlike
them 5 (2) that the germ-cells are very unlike ordinary
body-cells is shown by the fact that they can do what
no single body-cell can do, build up a whole organism ;
(3) so specific are germ-cells that in certain cases and
in favourable conditions a small fraction of an egg,
bereft of its own nucleus, may, if fertilised, develop
into an entire and normal larva; (4) it is quite con-
sistent with the idea of evolution that in lower
organisms the contrast between body-cells and germ-
cells should be less pronounced than in higher forms.
But the fundamental answer is found when we inquire
into the history of the germ-cells. In many cases,
and the list is being added to, the future reproductive
cells are segregated off at an early stage in embryonic
development. Even before differentiation sets in, the
future reproductive cells may be set apart from the
body-forming cells. The latter develop in manifold
variety into skin and nerve, muscle and blood, gut
and gland; they differentiate, and may lose almost all
protoplasmic likeness to the mother ovum. But the
reproductive cells are set apart; they take no share in
the differentiation, but remain virtually unchanged,
continuing unaltered the protoplasmic tradition of
the original fertilised ovum. After a while theirrms are organs of low