In humans, the transparent sac that contains fluid that cushions the embryo and maintains a constant temperature and pressure is the

It is a membrane called the amniotic sac or amnion. The amniotic sac encloses the fetus in the amniotic cavity, which contains amniotic fluid. The amniotic fluid supports and cushions the fetus before birth. It is also a shock absorber. As it cannot be compressed, it protects the fetus against physical injury. It allows the fetus a certain degree of movement, which promotes muscular development. During birth, it lubricated and reduces friction in the vagina or birth canal.