a bag of liquid that surrounds the embryo of a mammal, bird, or reptile inside its mother

In the centre of the amniotic sac, a little heart was beating away. 在羊膜囊的中心，跳动着一颗小小的心脏。

An amniotic sac contains a fetus during pregnancy. 怀孕期间，胎儿包裹在羊膜囊里。