Amniotic sac

It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth.

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.

[citation needed] The amniotic cavity is surrounded by an extraembryonic membrane, called the amnion.

As the implantation of the blastocyst progresses, a small space appears in the embryoblast, which is the primordium of the amniotic cavity.

Soon, amniogenic (amnion-forming cells) called amnioblasts separate from the epiblast and line the amnion, which encloses the amniotic cavity.

[citation needed] If, after birth, the complete amniotic sac or big parts of the membrane remain coating the newborn, this is called a caul.

The amniotic sac and its filling provide a liquid that surrounds and cushions the fetus.

An artificial rupture of membranes (AROM), also known as an amniotomy, may be clinically performed using an amnihook or amnicot in order to induce or to accelerate labour.

[10][11] This is fairly routine procedure, but can lead to infection of the amniotic sac in a very small number of cases.