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Amniotic sac

The amniotic sac, also known as the "bag of waters", is a transparent fluid filled sac in which the embryo/fetus develops. The amniotic sac is initially formed by the amnion, which is derived from the epiblast cells of the blastocyst, and is one of the four extraembryonic membranes that forms during the first two to three weeks of development. The amniotic membrane surrounds the embryo to form the sac, and extends along the connecting stalk to form the external covering of the umbilical cord.

The space within the sac in which the embryo develops is the amniotic cavity, which is filled with amniotic fluid. This fluid provides a buoyant medium to support the embryo, protecting against injury by providing cushioning and allowing the fetus to move around freely in the later stages of pregnancy

As the volume of the amniotic fluid increases during pregnancy, the amniotic membrane expands and attaches to the inner surface of the chorion (the outermost of the extraembryonic membranes), which then forms the outer layer of the amniotic sac.

Terminology English : Amniotic sac
Latin : Saccus amnii
Structure Fluid filled sac comprised of an inner amnion membrane layer and outer chorion membrane layer
Functions Protection of embryo/fetus

Learn more about fetal development with this study unit:

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