The amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They include mammals, birds and reptiles, as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In humans, these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes, and the lack of a larval stage, distinguish amniotes from tetrapod amphibians.