The membrane that forms a protective sac surrounding the embryo is called the amnion.

Here's why:

* Amnion: This membrane encloses the embryo in a fluid-filled sac called the amniotic sac. This fluid cushions and protects the developing embryo from shock and provides a stable temperature environment.

* Chorion: This membrane surrounds the amnion and helps with gas exchange and waste removal.

* Yolk sac: This membrane provides nourishment to the embryo in early development.

* Allantois: This membrane stores waste products and later contributes to the formation of the umbilical cord.

Let me know if you have any other questions.