aking them into his buccal cavity.

=Reproduction in Vertebrates.=--We should never finish if we were to
describe even the chief varieties of sexual union among the
vertebrates. As a rule, the male possesses a copulating organ which
projects externally, while the female presents an invaginated cavity,
more or less cylindrical, into which the male organ can penetrate. A
certain amount of sperm is deposited by the male in the neighborhood
of the mature ovules (Fig. 18) discharged from the female germinal
gland or ovary, which renders conjugation possible. By means of their
mobile tails, the spermatozoa (Fig. 11) are able to reach the ovules
and fecundate them. The manner in which the egg when fecundated,
either in the mother's body or after being laid, continues its
development, varies enormously in different species. The eggs are
often deposited by the female and the embryo develops outside the
mother's body. This occurs in insects, mollusks, fish, amphibia,
reptiles, birds and the lowest mammals or monotremes (ornithorhynchus
and echidna).

In the lower mammals is developed an organ called the womb which
allows the embryo to remain longer in the maternal body. This organ is
very incomplete in them, and a pocket or fold in the skin of the belly
allows the mother to carry her young, which are extremely embryonic at
birth, till they have developed sufficiently to live alone. This
occurs in marsupials (kangaroos and opossums), in which the vagina and
uterus are double.

In the higher mammals the womb becomes more and more developed,
opening into a single vagina in the middle line of the abdomen,
between the two ovaries, and constituting a highly specialized organ
which allows the mother to preserve the young for a long time in her
belly. In most mammals the uterus has two elongated diverticula, each
of which may contain a successive series of embryos. In man it forms a
single cavity and normally contains a single embryo, occasionally two
or more. These facts show that the role of the female mammal in
reproduction is more important than that of the male. But this is not
all. Whether it still lays eggs, or whether it gives birth to young
which are more or less developed its sexual role is far from ended.
The higher oviparous vertebrates, especially the birds, take care of
their progeniture for some time after laying. The young are still fed
by the mother, either by milk from the teats, as in mammals, or by
nourishment obt