extra-embryonic membrane surrounding the embryo, filled with fluids, some nutrients, and proteins absorbed from albumen; embryo floats in the amniotic fluid, which keeps it moist, protected, and cushioned and allows it to move and grow; has muscles that gently contract to prevent the parts of the embryo from adhering to each other; develops along with the chorion out of the ectoderm and mesoderm (germ layers that develop into epidermis, muscles, blood vessels, and organs) in the gut area of the embryo about 4 days into incubation, expands around and behind the embryo, fully surrounding it about a third of the way into incubation; chick swallows remaining amniotic fluid before hatch