▪ animal group
  a group of limbed vertebrates (vertebrate) that includes all living reptiles (reptile) (class Reptilia), birds (bird) (class Aves), mammals (mammal) (class Mammalia), and their extinct relatives and ancestors. The amniotes are the evolutionary branch (clade) of the tetrapods (tetrapod) (superclass Tetrapoda) in which the embryo develops within a set of protective extra-embryonic membranes (membrane)—the amnion, chorion, and allantois.

      The amnion, chorion, and allantois of amniotes likely evolved from the embryonic tissue layer encasing a large yolk mass. In living reptiles, a sheet of cells (cell) grows outward from the embryo. This growth, in combination with the growth and torsion of the embryo itself, causes the external layers of this tissue sheet to fold over the embryo from tail to head. When these folds meet above the embryo, they fuse. The tissue sheet, which becomes a tough, fluid-filled sac called the amniotic membrane, is made up of an outer chorion layer and an inner amnion layer. The function of the amniotic membrane is largely protective, whereas the allantois serves as a collection area for waste materials and a site of gas exchange. The allantois forms as an outpocketing of the embryo's hindgut and grows outward into the space between the amnion and chorion.

      This arrangement of extra-embryonic membranes is an evolutionary adaptation that permitted the deposition of eggs (egg) in terrestrial environments. As a result of this change, tetrapods could become fully terrestrial. In reptiles and birds, the amnion, chorion, and allantois are further protected by a hard or leathery calcareous shell. In most mammals and some live-bearing lizards (lizard), a portion of the amniotic membrane and the allantois form the placenta, a vascular organ that facilitates the exchange of nutrients (nutrient) and waste products between the growing fetus and its mother.

      Vertebrates that develop without the extra-embyronic membranes are known as anamniotes. They do not belong to a formal classification group, because they are associated by the absence of a characteristic. Among the tetrapods, anamniotes include extant and extinct amphibians (amphibian) and two groups of extinct reptilelike animals, the anthracosaurs (Diplovertebron) and the batrachosaurs.

George R. Zug

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Universalium. 2010.

Look at other dictionaries:

  • Amniota — Am ni*o ta, n. pl. [NL. See {Amnion}.] (Zo[ o]l.) That group of vertebrates which develops in its embryonic life the envelope called the amnion. It comprises the reptiles, the birds, and the mammals. [1913 Webster] …   The Collaborative International Dictionary of English

  • Amniota — Amniota, höhere Wirbeltiere, zusammenfassende Bezeichnung für die Wirbeltierklassen Reptilia, Aves und Mammalia. Diese Gruppen bilden ein Amnion und andere ⇒ Embryonalhüllen aus. Die beiden vordersten Wirbel der Wirbelsäule sind ⇒ Atlas und… …   Deutsch wörterbuch der biologie

  • amniotă — amniótă s. f., pl. (sil. ni o ) amnióte Trimis de siveco, 10.08.2004. Sursa: Dicţionar ortografic …   Dicționar Român

  • amniota — m. Zool. Vertebrado cuyo embrión desarrolla un amnios y una bolsa alantoidea, como ocurre en los reptiles, las aves y los mamíferos. U. m. en pl.) …   Diccionario de la lengua española

  • Amniota — Amniotes U …   Wikipédia en Français

  • Amniota —   Amniota Rango temporal: Carbonífero Presente …   Wikipedia Español

  • Amniota — ► sustantivo masculino ZOOLOGÍA Vertebrado que tiene el embrión protegido por un amnios. SINÓNIMO anamniota * * * amniota. m. Zool. Vertebrado cuyo embrión desarrolla un amnios y una bolsa alantoidea, como ocurre en los reptiles, las aves y los… …   Enciclopedia Universal

  • Amniota — Amnioten Aus dem Ei schlüpfende Schildkröte Systematik ohne Rang: Neumünder (Deuterostomia) Stamm …   Deutsch Wikipedia

  • Amniota — …   Википедия

  • Amniota — noun The reptiles, birds and mammals, collectively, all of which have an amnion during development …   Wiktionary

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