minal Cells. Hermaphrodites.= We now come to _conjugation_. In
order to avoid complications we will leave aside plants and speak only
of animals. Among multicellular animals, sometimes in the same
individual, sometimes in different individuals, occur two kinds of
sexual glands, each containing one kind of cells--the male cells and
the female cells. When both kinds of sexual glands occur in the same
individual, the animal is said to be _hermaphrodite_. When they
develop in two different individuals the animals are of distinct
sexes. Snails, for example, are hermaphrodite. There also exist lower
multicellular animals which reproduce by budding, but among which
conjugation takes place from time to time. We shall not consider these
animals any further, as they are too remote to interest us here.

=Spermatozoa and Ova.=--In all the higher animals, including the
hermaphrodites, the male germinal cells, or _spermatozoa_ are
characterized by their mobility. Their protoplasm is contractile and
their form varies according to the species. In man and vertebrate
animals they resemble infinitely small tadpoles, and their tails are
equally mobile. The female germinative cell, on the contrary, is
immobile and much larger than the male cell. Conjugation consists in
the movement of the male cell, by means of variable mechanism, toward
the female cell, or egg, into the protoplasm of which it enters. At
this moment it produces on the surface of the egg a coagulation, which
prevents the entrance of a second spermatozoid.

The egg and the spermatozoid both consist of protoplasm containing a
nucleus. But, while the spermatozoid has only a small nucleus and very
little protoplasm, the egg has a large nucleus and a large quantity of
protoplasm. In certain species the protoplasm of the egg grows in the
maternal organism in a regular manner to form the _vitellus_ (yolk of
egg) which serves as nourishment for the embryo for a long period of
its existence. This occurs in birds and reptiles.

=Conjugation.=--The phenomena of conjugation were made clear by _van
Beneden_ and _Hertwig_. These phenomena, as we have seen, commence
among unicellular organisms. In these they do not constitute
reproduction, but the vital reenforcement of certain individuals.
Conjugation takes place in a different manner in different cases.

For example, a unicellular animal applies itself against one of its
fellows. The nucleus of each cell divides into two. Then the
pr