l birth, that is about nine
months (ten lunar months of four weeks). The embryo is then ready to
separate from the maternal body (Fig. 22). By the act of birth it is
expelled violently, bringing with it the umbilical cord and the
placenta (Fig. 23). Immediately afterward the empty womb contracts
strongly and gradually recovers its former size. The sudden
interruption of its communications with the maternal circulation
deprives the embryo, which has suddenly become a child, of its
nutritive matter and oxygen.

[Illustration: FIG. 23. Sagittal section of frozen body of a
woman in labor: the head of the child is engaged in the neck of
the womb; the orifice of the neck of the womb (_os uteri_) is
already fully dilated and the bag of waters commences to project
from the vulva: it is formed by the former membranes of the egg
and the decidua.]

In order to avoid suffocation it is obliged to breathe atmospheric air
immediately, for its blood becomes dark by saturation with carbonic
acid, which irritates the respiratory nerve centers. The first
independent act of the new-born child is, therefore, a nervous reflex
determined by asphyxia, and is performed with the first cry. Soon
afterward the infant begins to suck, so as not to die of hunger, while
the umbilical cord, having become useless, shrivels up, and the
placenta is destroyed (some animals eat it). The new-born infant is
only distinguished from the embryo soon after birth by its breathing
and crying.

We may, therefore, say that infancy, especially early infancy, is only
a continuation of embryonic life. The transformations which the infant
undergoes from birth to adult age are known to all. They take place
more and more slowly, except at the relatively short period of
puberty.

=Formation of the sexual glands.=--We must remember that at a very
early embryonic period certain groups of cells are reserved to form
later on the sexual glands. These cells are at first neither male nor
female, but are undifferentiated; later on they become differentiated
to form in certain individuals, called males, the testicles with their
spermatozoa, and in others, called females, the ovaries with their
eggs. On this differentiation depends the sex of the individual, and,
according as it takes place in one way or the other, all the rest of
the body develops with the correlative sexual characters of the
corresponding sex (at first the external genital organs peculiar to