It is the amniotic egg. This kind of egg is unique to the amniotes, a group that includes turtles, lizards, birds, dinosaurs, and mammals. The structure of the amniote egg is such that inside the egg are a series of fluid-filled membranes which permit the embryo to survive: the amnion, allantois, yolk sac, and chorion. Surrounding and protecting the embryo is the amnion, filled with amniotic fluid, and providing the embryo with a stable fluid environment.