The
amniotic sac, commonly called the
bag of waters, sometimes the
membranes, is the sac in which the
fetus develops in
amniotes. It is a thin but tough transparent pair of
membranes that hold a developing
embryo (and later fetus) until shortly before
birth. The inner of these
fetal membranes, the
amnion, encloses the
amniotic cavity, containing the
amniotic fluid and the fetus. The outer membrane, the
chorion, contains the amnion and is part of the
placenta. On the outer side, the amniotic sac is connected to the
yolk sac, to the
allantois and, through the
umbilical cord, to the
placenta.