page_title: Amniote
section_title: Description
hierarchical_section_title: Amniote / Description
context_page_description: Amniotes are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes lay their eggs on land or retain the fertilized egg within the mother, and are distinguished from the anamniotes, which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the evolutionary great chain of being.
  Amniotes are tetrapods that are characterised by having an egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anamniotes typically do. Amniotes include synapsids and sauropsids, as well as their ancestors, back to amphibians. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals, 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.
context_section_description: Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis, as amphibians do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis. The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size.
caption: Anatomy of an amniotic egg: EggshellOuter membraneInner membraneChalazaExterior albumen (outer thin albumen)Middle albumen (inner thick albumen)Vitelline membraneNucleus of PanderGerminal disk (blastoderm)Yellow yolkWhite yolkInternal albumenChalazaAir cellCuticula