ovum. These features are of great importance not only as regards
conception itself, but for the development of the organic form, and
especially for the differentiation of the sexes. There is a
particularly curious correlation of plants and animals in this
respect. The splendid studies of Charles Darwin and Hermann Muller on
the fertilisation of flowers by insects have given us very interesting
particulars of this.* (* See Darwin's work, On the Various
Contrivances by which Orchids are Fertilised (1862).) This reciprocal
service has given rise to a most intricate sexual apparatus. Equally
elaborate structures have been developed in man and the higher
animals, serving partly for the isolation of the sexual products on
each side, partly for bringing them together in conception. But,
however interesting these phenomena are in themselves, we cannot go
into them here, as they have only a minor importance--if any at
all--in the real process of conception. We must, however, try to get a
very clear idea of this process and the meaning of sexual
reproduction.

In every act of conception we have, as I said, to consider two
different kinds of cells--a female and a male cell. The female cell of
the animal organism is always called the ovum (or ovulum, egg, or
egg-cell); the male cells are known as the sperm or seed-cells, or the
spermatozoa (also spermium and zoospermium). The ripe ovum is, on the
whole, one of the largest cells we know. It attains colossal
dimensions when it absorbs great quantities of nutritive yelk, as is
the case with birds and reptiles and many of the fishes. In the great
majority of the animals the ripe ovum is rich in yelk and much larger
than the other cells. On the other hand, the next cell which we have
to consider in the process of conception, the male sperm-cell or
spermatozoon, is one of the smallest cells in the animal body.