Amniote

An amniote is a tetrapod vertebrate whose embryo develops within an amnion, an extraembryonic membrane that helps protect it from drying out. Amniotes include mammals, reptiles, and birds.

Amnion: A membrane surrounding the embryo that helps create a fluid-filled environment during development. This defining membrane gives amniotes their name and protects embryos on land.
Amniotic egg: An egg containing an embryo enclosed by extraembryonic membranes, including an amnion. Its membranes enable development on land without requiring an aquatic breeding site.
Amniote egg fossil: A fossilized amniote egg or eggshell that preserves evidence of reproduction in the geological past. Such fossils provide direct, though rare, evidence of early amniote reproduction.
Amphibian: A tetrapod vertebrate whose life cycle commonly includes aquatic eggs or larvae and permeable skin. Most amphibians lack an amnion and remain more dependent on moist environments for reproduction.
Chorion: An outer extraembryonic membrane involved in gas exchange and, in mammals, placental development. It works alongside the amnion in amniote development.
Internal fertilization: Fertilization in which sperm and egg unite inside the reproductive tract of the female. It allows amniote reproduction to avoid the external fertilization common in many amphibians.
Hylonomus: An extinct early amniote known from Late Carboniferous fossils in Nova Scotia. Its fossils document one of the earliest known animals commonly identified as an amniote.
Anamniote: A vertebrate whose embryo does not develop within an amnion, a grouping that includes fishes and amphibians. This category identifies vertebrates outside the amniote reproductive lineage.
Allantois: An extraembryonic membrane that stores waste in many embryos and contributes to gas exchange or the placenta. This membrane is part of the amniote egg’s developmental toolkit.
Placenta: An organ formed between maternal and embryonic tissues that exchanges nutrients, gases, and wastes during pregnancy. In placental mammals, it adapts amniote membranes to development inside the mother.

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