The amniotic sac differs between reptiles, birds, and mammals in terms of structure and function. In reptiles, the amniotic sac is a thin membrane that surrounds the developing embryo and is filled with fluid. In birds, the amniotic sac is more complex and consists of two layers: the amnion, which directly surrounds the embryo, and the chorion, which lines the inside of the eggshell. This structure allows for efficient gas exchange during incubation. In mammals, the amniotic sac forms the amniotic cavity, which contains the embryo and is filled with amniotic fluid. The amnion, a thin membrane, encloses the amniotic cavity and protects the developing fetus. Mammals also have a specialized organ called the placenta, which provides nutrients and oxygen to the developing fetus and removes waste products.