mals are still more various and wonderful than in plants.
In some of the lower animals, as in most fish and reptiles, both elements
are discharged from the bodies of the parents before coming in contact,
there being no contact of the two individuals. In this class of animals
the process is almost wholly analogous to fecundation in those plants
in which the male and female flowers are on different plants or
different parts of the same plant. In the female fish, a large number
of ova are developed at a certain season of the year known as the
spawning season. Sometimes the number reaches many thousands. At the
same time, the testicles of the male fish, which are contained within
the abdominal cavity, become distended with developed zoosperms. When
the female seeks a place to deposit her eggs, the male closely follows;
and as she drops them upon the gravelly bottom, he discharges upon them
the zoosperms by which they are fecundated. The process is analogous
to some species of frogs. When the female is about to deposit her eggs,
the male mounts upon her back and rides about until the eggs are all
deposited, discharging upon them the fertilizing spermatozoa as they
are laid by the female.

In higher orders of animals, fecundation takes place within the
generative organs of the female by contact between the male and the
female organs. To effect this, there are necessitated certain accessory
organs, the _penis_ in the male and the _vagina_ in the female.

Nothing in all the range of nature is more remarkable than the
adaptation of the two varieties of sexual organs in each species. This
necessary provision is both a powerful means of securing the
perpetuation of the species, and an almost impassable barrier against
amalgamation.

The act of union, or sexual congress, is called _coitus_ or _copulation_.
It is accompanied by a peculiar nervous spasm due to excitement of
special nerves principally located in the _penis_ in the male, and in
an extremely sensitive organ, the _clitoris_, in the female. The
nervous action referred to is more exhausting to the system than any
other to which it is subject.

Union of the Ovum and Zoosperm.--The zoosperms not only come in contact
with the ovum, but penetrate the thin membrane which incloses its
contents, and enter its interior, where they disappear, becoming united
with its substance. In the ova of certain fishes, small openings have
been observed through which the spermatozoa find en