(Very highly magnified.)
The microscopic animal Trypanosome, which causes Sleeping Sickness. The
study of these organisms has of late years acquired an immense
importance on account of the widespread and dangerous maladies to which
some of them give rise. It lives in the blood of man, who is infected by
the bite of a Tse-tse fly which carries the parasite from some other
host.]
[Illustration: VOLVOX
The Volvox is found in some canals and the like. It is one of the first
animals to suggest the beginning of a body. It is a colony of a thousand
or even ten thousand cells, but they are all cells of one kind. In
_multicellular_ animals the cells are of _different_ kinds with
different functions. Each of the ordinary cells (marked 5) has two
lashes or flagella. Daughter colonies inside the Parent colony are being
formed at 3, 4, and 2. The development of germ-cells is shown at 1.]
[Illustration: PROTEROSPONGIA
One of the simplest multicellular animals, illustrating the beginning of
a body. There is a setting apart of egg-cells and sperm-cells, distinct
from body-cells; the collared lashed cells on the margin are different
in kind from those farther in. Thus, as in indubitable multicellular
animals, division of labour has begun.]
Splitting into two or many parts was the old-fashioned way of
multiplying, but one of the great steps in evolution was the discovery
of a better method, namely, sexual reproduction. The gist of this is
simply that during the process of body-building (by the development of
the fertilised egg-cell) certain units, _the germ-cells_, do not share
in forming ordinary tissues or organs, but remain apart, continuing the
full inheritance which was condensed in the fertilised egg-cell. _These
cells kept by themselves are the originators of the future reproductive
cells of the mature animal_; they give rise to the egg-cells and the
sperm-cells.
The advantages of this method are great. (1) The new generation is
started less expensively, for it is easier to shed germ-cells into the
cradle of the water than to separate off half of the body. (2) It is
possible to start a great many new lives at once, and this may be of
vital importance when the struggle for existence is very keen, and when
parental care is impossible. (3) The germ-cells are little likely to be
prejudicially affected by disadvantageous dints impressed on the body of
the parent--little likely unless the dints have peculiarly penetrating
consequences, as in the case of poisons. (4) A further advantage is
implied in the formation of two kinds of germ-cells--the ovum or
egg-cell, with a considerable amount of building material and often with
a legacy of nutritive yolk; the spermatozoon or sperm-cell, adapted to
move in fluids and to find the ovum from a distance, thus securing
change-provoking cross-fertilisation.
§ 4