placenta. The cord contains three blood vessels—two
—that transport blood
between the embryo and the placenta (f g. 23.17). The umbili-
cal cord also suspends the embryo in the
In addition to the amnion and chorion, two other extra-
embryonic membranes Form during development. They are
the yolk sac and the allantois.
Forms during the second week, and it is
attached to the underside oF the embryonic disc (see Fig.
23.15). This structure Forms blood cells in the early stages
oF development and gives rise to the cells that later become
sex cells. The yolk sac also produces stem cells oF the bone
marrow, which are precursors to many cell types, but pre-
dominantly to blood cells. Parts oF the yolk sac Form the
embryonic digestive tube as well. Part oF the membrane
derived From the yolk sac becomes incorporated into the
umbilical cord, and the remainder lies in the cavity between
the chorion and the amnion near the placenta.
to-is) Forms during the third week
as a tube extending From the early yolk sac into the connecting
stalk oF the embryo. It, too, Forms blood cells and gives rise to
the umbilical arteries and vein (see f gs. 23.15 and 23.17).
Eventually, the amniotic cavity becomes so enlarged that
the membrane oF the amnion contacts the thicker chorion
around it. The two membranes Fuse into an
By the beginning oF the eighth week, the embryo is
usually 30 millimeters long and weighs less than 5 grams.
Although its body is unf
nished, it looks human
(f g. 23.19)
the woman’s body, and maintains a stable temperature For
proper embryonic and Fetal development.
Certain epithelial cells of the amnion can be cultured in laboratory
dishes with biochemicals that coax them to become pluripotent stem
cells. Changing the culture medium stimulates these cells to diF
entiate as cartilage, bone, fat, muscle, pancreas, liver, or neuron—
representatives of all three primary germ layers. These amniotic cells
are just one example of stem cell sources from what would otherwise
be medical waste.
The developing placenta synthesizes progesterone From
cholesterol in the maternal blood. Cells associated with
the developing Fetal adrenal glands use the placental pro-
gesterone to synthesize estrogens. The estrogens, in turn,
promote changes in the maternal uterus and breasts and
inFluence maternal metabolism and the development oF
As the embryo becomes more cylindrical, the margins
oF the amnion Fold, enclosing the embryo in the amnion and
amniotic ﬂ uid. The amnion envelopes the tissues on the under-
side oF the embryo, particularly the connecting stalk, by which
it is attached to the chorion and the developing placenta. In
this manner, the
ı˘-kal kord) Forms (see
f g. 23.15).
The Fully developed umbilical cord is about 1 centimeter
in diameter and about 55 centimeters in length. It originates at
the umbilicus oF the embryo and inserts into the center oF the
membrane forms from fusion of the
amnion and the chorion.