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noun, plural: amniotes

Any of the tetrapod vertebrates belonging to Amniota in which an amnion is formed during its embryonic or fetal stage


Amniotes belong to a clade of animals, Amniota, to distinguish them from other vertebrates that are referred to as anamniotes. The amniotes have an amnion during their embryonic or fetal stage in contrast to anamniotes that do not. Amnion is one of the extraembryonic membranes that form and surround the embryo or fetus. The amnion is a thin and a tough membrane and lines the amniotic cavity. It is comprised of two cell layers: the epiblast-derived extraembryonic ectodermal layer and the thin non-vascular extraembryonic mesoderm. It is derived from trophoblasts by folding or splitting.1 Together with the chorion, the amnion forms the amniotic sac. Inside the sac is an amniotic fluid that keeps the fetus afloat and thereby preventing physical injuries. The amnion also serves to transport oxygen into the egg as well as expel carbon dioxide.

Examples of amniotes are reptiles, birds, and mammals. The reptiles and birds lay eggs on land where the latter hatch in time. The mammals retain their fertilized egg inside the uterus where the embryo develops into a fetus and is delivered alive at birth.

Word origin: Greek amnion, amnos (lamb)

Scientific classification:


See also:

1 Amnion. Dictionary. Retrieved from [[1]].

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