fetus and forms a closed sac around it filled with a fluid known as liquor
amnii, in which the fetus floats. The allantois is composed of two layers,
which form a closed sac in connection with the urachus, or the tube which
extends from the fetal bladder through the umbilical cord. The one layer of
the allantois is spread over the outer surface of the amnion and the other
over the inner surface of chorion. The allantois also contains a fluid
which is known as the allantoid liquid. The chorion is the outer envelope
or membrane of the fetus, completely inclosing the fetus with its other
membranes. On the outer surface of this membrane are found the fetal
placentulae, or cotyledons, which, through their attachment to the maternal
cotyledons, furnish the fetus with the means of sustaining life. The
relation of the fetal and maternal cotyledons to each other is illustrated
on the following plate.

PLATE XIII. Pregnant uterus with cotyledons.

Fig. 1. Uterus of the cow during pregnancy, laid open to show the
cotyledons (_d_) on the internal surface of uterus (_c_). The ovary (_a_)
is shown cut across, and the two halves are laid open to show the position
of the discharged ovum at _a_'.

Fig. 2 illustrates the relation of the fetal and maternal parts of a
cotyledon. A portion of the uterus (_A_) is shown with the maternal
cotyledon (_BB_) attached to it. The fetal portion (_D_) consists of a mass
of very minute hairlike processes on the chorion (_E_), which fit into
corresponding depressions or pits of the maternal portion. Each portion is
abundantly supplied with blood vessels, so that a ready interchange of
nutritive fluid may take place between mother and fetus.

PLATE XIV. Vessels of umbilical cord.

Fig. 1. Fetal calf with a portion of the wall of the abdominal cavity of
the right side and the stomach and intestines removed to illustrate the
nature of the umbilical or navel cord. It consists of a tube (1-1') into
which pass the two umbilical arteries (3) carrying blood to the placenta in
the uterus or womb and the umbilical vein (4) bringing the blood back and
carrying it into the liver. The cord also contains the urachus (2') which
carries urine from the bladder (2) through the cord. These vessels are all
obliterated at birth. 5, liver; 5', lobe of same, known as the lobus
Spigelii; 5'', gall bladder; 6, right kidney; 6', left kidney; 6'',
ureters, or the tubes conducting the urine from the kidneys to the bladder;