Amniotic sac

The amnion (: amnions or amnia) is a membrane that closely covers human and various other embryos when they first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect the embryo. In birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus.

the fluid surrounding a fetus within the amnion

The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs.

The allantois ( ; : allantoides or allantoises) is one of the four membranes of the extraembryonic tissue arising from the yolk sac. It is a hollow sac-like structure filled with clear fluid that forms part of the developing conceptus in an amniote that helps the embryo exchange gases and handle liquid waste. The other extraembryonic membranes are the yolk sac, the amnion, and the chorion. In mammals these membranes are known as fetal membranes.

sac in which the fetus develops in amniotes

thumb|Hippomanes of a cow Hippomanes (Greek ἱππομανές) are brownish or olive-greenish, elastic formations that arise physiologically in female bovines and equines during pregnancy.