RETENTION OF THE FETAL MEMBRANES.--The foetus is enveloped by several
layers of membranes. The _external envelope, the chorion_, is exactly
adapted to the uterus. The _innermost envelope, the amnion_, encloses the
foetus. Covering the external face of the amnion and lining the inner face
of the chorion is a double membrane, _the allantois_. The envelopes
mentioned are not the only protection that the foetus has against injury.
It is enveloped in fluids as well. Immediately surrounding it is the
_liquor amnii_, and within allantois is the _allantoic fluid._

[Illustration: FIG. 19.--Placenta of cow.]

_The placenta_ is a highly vascular structure spread out or scattered over
the surface of the chorion and the mucous membrane of the uterus, that
attaches the foetus and its envelopes to the womb (Fig. 19). It is by means
of this vascular apparatus that the foetus is furnished with nourishment.
The fetal and maternal placentas are made up of vascular villi and
depressions that are separated only by the thin walls of capillaries, and a
layer of epithelial cells. This permits a change of material between the
fetal and maternal circulation. The arrangement of the placenta differs in
the different species. In the mare and sow, the villi are diffused. In
ruminants, the villi are grouped at certain points. These vascular masses
are termed cotyledons. The maternal cotyledons or "buttons" form appendages
or thickened points that become greatly enlarged in the pregnant animal.

Toward the end of the pregnant period, the attachments between the fetal
and maternal placentulae undergo a fatty degeneration and finally separate.
This results in contractions of the muscular wall of the uterus, and the
expulsion of the foetus and its envelopes. In the mare, it is not uncommon
for the colt to be born with the covering intact. This does not occur in