An amniotic egg is an egg that contains amniotic fluid and a yolk to nourish a developing organism. The outer shell of the amniotic egg serves the purpose of protection. Moreover, the structure and material of the egg aid the organism in gas and energy exchange. Amniotic eggs are usually laid by mammals, reptiles, birds, and other organisms classified as amniotes, a clade of tetrapods. Scientists and researchers often claim that the adaptation of the amniotic egg represents the divergence of vertebrates. In addition, scientists believe the adaption of amniotic eggs has led to the distribution of many vertebrate species.