, and muscle. Then in the
springtime they grow rapidly. They grow until they are ripe, when they
lie free in the ovary; and others grow and are freed in the same way
until the ovary, which has also enlarged to accommodate them, is quite
full. The female fish is larger than the male, and looks plump and
rounded at this season. In course of time the eggs thus developed will
be shed--or born--whether they are fertilized or not. But, if they are
not fertilized, no further growth will take place in them, and they will
soon perish.

The child, knowing about the fertilization of flowers, can easily be led
to see that the fish ova, like the flower ovules, cannot develop without
pollen. The anthers containing the pollen are found in the male fish,
and look like the ovaries, only they are not so large and their contents
are not so firm. They seem filled with a formless substance instead of
with little globular eggs. Under the microscope this formless substance
is seen to be made of a semi-fluid material in which are held millions
of pollen grains! Only we no longer call them pollen grains. We may call
them fertilizing cells if we please, though there are several names for
them. But they are essentially the same as pollen. They grow, in the
same way, from the inside of the anther (which may now be called the
testicle) and become free when ripe. The pollen grains cannot move of
themselves; the fertilizing cells can. Each fertilizing cell is like an
ovum, excepting that it is not so spherical and is lengthened into a
sort of lash by which it can propel itself through the water. When the
ova are laid by one fish, the other swims over them and the fertilizing
fluid is expelled into the water just as the eggs were. There is no
union whatever between the parents for the purpose of fertilization. As
soon as a fertilizing cell comes in contact with an ovum it seeks to
enter into its substance, and as soon as this has happened, the two
cells thus united begin to develop into a very tiny fish. As soon as the
change begins, we have the _embryo_ of the fish, which thus corresponds
to the embryo of the seed.

There is one great difference between the ovary of the plant and that of
the fish. When the plant ovary is ripe, its seeds are shed, and then the
ovary itself falls off. The plant ovary thus bears only one set of
seeds. In the fish, the ovary always remains in the fish, and after the
eggs are shed, it shrinks up to a very small size, and