Answer the following question given this paragraph: 

Attached to the placenta is the amniotic sac , an enclosed membrane that surrounds and protects the fetus. It contains amniotic fluid , which consists of water and dissolved substances. The fluid allows the fetus to move freely until it grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury.


Q: What is the enclosed, fluid-filled membrane that surrounds and protects the fetus and is attached to the placenta?


A:
amniotic sac