Merriam-Webster Collegiate Dictionary - amnion

 noun  (plural ~s or amnia)  Etymology: New Latin, from Greek, caul, from amnos lamb — more at yean  Date: 1667  1. a thin membrane forming a closed sac about the embryos or fetuses of reptiles, birds, and mammals and containing the amniotic fluid  2. a membrane analogous to the ~ and occurring in various invertebrates  • amniotic adjective