The male sexual cells are always of microscopic size and are produced in
the generative gland or testis in exceedingly large numbers. In addition to
their minuter size they differ from the ova in their power of active
movement. Animals present various mechanisms by which the sexual elements
may be brought into juxtaposition, but in all cases some distance must be
traversed in a fluid or semifluid medium (frequently within the body of the
female parent) before the necessary fusion can occur. To accomplish this
latter end of its journey the spermatozoon is endowed with some form of
motile apparatus, and this frequently takes the form of a long flagellum,
or whip-like process, by the lashing of which the little creature propels
itself much as a tadpole with its tail.

In plants as in animals the female cells or ovules are larger than the
pollen grains, though the disparity in size is not nearly so marked. Still
they are always relatively minute cells since the circumstances of their
development as parasites upon the mother plant render it unnecessary for
them to possess any great supply of food yolk. The ovules are found
surrounded by maternal tissue in the ovary, but through the stigma and down
the pistil a {4} potential passage is left for the male cell. The majority
of flowers are hermaphrodite, and in many cases they are also
self-fertilising. The anthers burst and the contained pollen grains are
then shed upon the stigma. When this happens, the pollen cell slips through
a little hole in its coat and bores its way down the pistil to reach an
ovule in the ovary. Complete fusion occurs, and the minute embryo of a new
plant immediately results. But for some time it is incapable of leading a
separate existence, and, like the embryo mammal, it lives as a parasite
upon its parent. By the parent it is provided with a protective wrapping,
the seed coat, and beneath this the little embryo swells until it reaches a
certain size, when as a ripe seed it severs its connection with the
maternal organism. It is important to realise that the seed of a plant is
not a sexual cell but a young individual which, except for the coat that it
wears, belongs entirely to the next generation. It is with annual plants in
some respects as with many butterflies. During one summer they are
initiated by the union of two sexual cells and pass through certain stages
of larval development--the butterfly as a caterpillar, the plant as a
parasite upon i