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amniotic sac

Tuesday 6 April 2004


Definition : The amniotic sac is the sac in which the embryo and later 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 membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the embryo.

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, the allantois, and via the umbilical cord, the placenta.

Extraembryonic membranes

The yolk sac, amnion, chorion, and allantois are the four extraembryonic membranes that lie outside of the embryo and are involved in providing nutrients and protection to the developing embryo.

They form from the inner cell mass; the first to form is the yolk sac followed by the amnion which grows over the developing embryo.

The amnion remains an important extraembryonic membrane throughout prenatal development.

The third membrane is the allantois, and the fourth is the chorion which surrounds the embryo after about a month and eventually fuses with the amnion.


- Amniotic sac diseases

  • amniotic sac infections