of the bundle is ruptured, thus liberating the young
spermatozoa, which speedily complete their full development. The
spermatozoon is pure protoplasm, which is the basis of all life, and
its power of spontaneous motion is due to this fact.

In man, the formation of spermatozoa continues with greater or less
rapidity from puberty to old age, though at the two extremes of
existence they are imperfectly developed. When not discharged from the
body, they are said to be absorbed. Some physiologists claim that they
are composed of a substance identical with nerve tissue, and that by
absorption they play a very important part in the development and
maintenance of the nervous system.

It is asserted by good authorities that the reproductive element in
man is not so well developed as to be really fit for the reproduction
of the species before the age of twenty-four or twenty-five. After the
age of forty-five or fifty, the reproductive elements deteriorate in
quality, and become again unfitted for vigorous procreation.

The fully developed zoosperms are suspended in a transparent,
gelatinous fluid, which, mingled with the secretion of the prostate
gland and other fluids which it meets during its expulsion from the
body, constitutes the _semen_.

The Ovum.--The female element of generation, the ovum, is produced by
an organ called the _ovary_, of which there are two in each individual.
In size and form, the ovary closely resembles the testicle. Like the
latter organ, also, it is formed within the body early in the process
of development; but instead of passing outward and downward, as does
the testicle, it remains within the abdominal cavity, suspended in
place by ligaments. It is connected with a duct which receives the ovum
as it is discharged, and conveys it to the uterus.

The human ovum varies in size from 1/240 to 1/120 of an inch in diameter,
and consists of a single cell. Ova are not formed in such large numbers
as zoosperms. As a general rule, in the human female, a single ovum
is developed and discharged once in about four weeks, during the period
of sexual activity.

Fecundation.--It is often asked, and the question has elicited some
discussion, Which is the principal reproductive element; the zoosperm,
or the ovum? The ancients supposed the male element to be the essential
element, being simply nourished and developed by the female; but modern
research in biological science does not sustain this view. Probably