here, only death by violence or unfavorable conditions. The protozoa
are immortal, not in the sense of the endless persistence of the
individual, but of the absence of death. Heredity is here easily
comprehensible, for one-half, or less frequently a smaller fraction,
of the substance of the parent goes to form the new individual.
There is direct continuity of substance from generation to
generation.

But in volvox a change has taken place. The fertilized egg-cell,
formed by the union of egg and spermatozoon, is a single cell, like
the individual resulting from the conjugation or fusion of two
protozoa. But in the many-celled individual, which develops out of
the fertilized egg, there are two kinds of cells. 1. There are other
egg-cells, like the first, each one of which can, under favorable
conditions, develop into a multicellular individual like the
parent. And the germ-cells (eggs and spermatozoa) of volvox are
immortal like the protozoa. But, 2, there are nutritive, somatic
cells, which nourish and transport the germ-cells, and after their
discharge die. These somatic cells, being mortal, differ altogether
from the germ-cells and the protozoa. The protoplasm must differ in
chemical, or molecular, or other structure in the two cases, and we
distinguish the germ-plasm of the germ-cells, resembling in certain
respects Nägeli's idioplasm, from somatoplasm, which performs most
of the functions of the cell. The somatoplasm arises from, and hence
must be regarded as a modification of, the germ-plasm. The
germ-plasm can increase indefinitely in the lapse of generations,
increase of the somatoplasm is limited.

When a new individual develops, a certain portion of the germ-plasm
of the egg is set aside and remains unchanged in structure. This,