Savage, Jay M. Department of Biology, University of Miami, Coral Gables, Florida.
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A collective term for the classes Reptilia (reptiles), Aves (birds), and Mammalia (mammals) of the subphylum Vertebrata. The remaining vertebrates, including the several classes of fishes and the amphibians, are grouped together as the Anamnia. Members of the Amniota are characterized by having a series of specialized protective extraembryonic membranes during development. Three of the membranes—amnion, chorion or serosa, and allantois—occur only in this group, but a fourth, the yolk sac, is sometimes present and is found in many anamniotes. The presence of the extraembryonic membranes makes it possible for the embryonic development of the amniotes to take place out of the water. In the most primitive forms the early stages of development take place inside a shell-covered egg that is deposited on land. This pattern is typical of most reptiles, all birds, and some mammals (Figs. 1 and 2). In these animals the amnion and chorion form fluid-filled sacs which protect the embryo from desiccation and shock. The allantois usually acts as a storage place for digestive and nitrogenous wastes and, in conjunction with the chorion, as a respiratory structure. In viviparous reptiles and mammals the chorion and allantois generally fuse and become more or less intimately associated with the uterine lining of the mother. Nutritive, excretory and respiratory exchanges take place across the chorioallantoic membrane between the allantoic circulation of the embryo and the uterine circulatory vessels of the mother. See also: Allantois; Amnion; Anamnia; Vertebrata; Yolk sac
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