Conception usually consists in the bringing into contact with the ovum
of a slimy fluid secreted by the male, and this may take place either
inside or out of the female body. This fluid is called sperm, or the
male seed. Sperm, like saliva or blood, is not a simple fluid, but a
thick agglomeration of innumerable cells, swimming about in a
comparatively small quantity of fluid. It is not the fluid, but the
independent male cells that swim in it, that cause conception.

(FIGURE 1.20. Spermia or spermatozoa of various mammals. The
pear-shaped flattened nucleus is seen from the front in I and sideways
in II. k is the nucleus, m its middle part (protoplasm), s the mobile,
serpent-like tail (or whip); M four human spermatozoa, A spermatozoa
from the ape; K from the rabbit; H from the mouse; C from the dog; S
from the pig.

FIGURE 1.21. Spermatozoa or spermidia of various animals. (From Lang).
a of a fish, b of a turbellaria worm (with two side-lashes), c to e of
a nematode worm (amoeboid spermatozoa), f from a craw fish
(star-shaped), g from the salamander (with undulating membrane), h of
an annelid (a and h are the usual shape).

FIGURE 1.22. A single human spermatozoon magnified 2000 times; a shows
it from the broader and b from the narrower side. k head (with
nucleus), m middle-stem, h long-stem, and e tail. (From Retzius.))

The spermatozoa of the great majority of animals have two
characteristic features. Firstly, they are extraordinarily small,
being usually the smallest cells in the body; and, secondly, they
have, as a rule, a peculiarly lively motion, which is known as
spermatozoic motion. The shape of the cell has a good deal to do with