The membrane that forms a sac around an animal embryo is called the amnion.

Here's some more detail:

* Amnion: This membrane is a thin, tough sac that surrounds the developing embryo, filled with amniotic fluid. The amniotic fluid cushions the embryo, protecting it from physical shocks and providing a stable environment for growth.

* Amniotic sac: This refers to the entire structure, including the amniotic fluid and the amnion itself.

* Amniotes: Animals that develop within an amnion, including reptiles, birds, and mammals.

Let me know if you have any more questions!