amnion

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A thin, tough, membranous sac that encloses the embryo or fetus of a mammal, bird, or reptile. It is filled with a serous fluid in which the embryo is suspended.

Century Dictionary and Cyclopedia

1. n. In anatomy and vertebrate zoöl., one of the fetal appendages; the innermost one of the membranes which envelop the embryo of the higher vertebrates, as mammals, birds, and reptiles; the lining membrane of a shut sac, familiarly called the “bag of waters,” in which the fetus is contained. An amnion is developed in those vertebrates only which have a fully formed allantois; hence it is absent in the Ichthyopsida, or fishes and amphibians, but present in all Sauropsida, or reptiles and birds, and in Mammalia. The amnion is formed, at a very early period in the life of the embryo, by a duplication of the epiblast, or external blastodermic membrane, which, carrying with it a layer of mesoblast from the somatopleural division of the latter, rises on all sides about the embryo, the folded edges coming together over the back of the embryo, and there coalescing to form a shut sac in which the embryo is inclosed. From this mode of growth, it is obvious that what was the outer side of the amniotic folds becomes the inner side of the sac when it has shut, so that the epiblastic layer is internal, the mesoblastic external; the process of inversion being comparable to that by which, in the case of the primitive trace of the embryo, a layer of epiblast is converted into the lining of the spinal canal. Only that fold of membrane which is next the body of the embryo composes the amnion proper, the other or outer fold in contact with the enveloping primitive chorion (vitelline membrane or yolk-sac) either disappearing or taking part in the formation of the permanent chorion. As long as this outer fold is recognizable as a membrane, it bears the name of false amnion. The shut sac of the amnion contains the liquor amnii, a bland, albuminous, serous fluid in which the fetus is immersed. In parturition, rupture of the sac is followed by the “bursting of the waters.” Sometimes a portion of the sac adheres to the head of the child, fitting like a skull-cap; such an infant is said, in the language of midwives, who commonly regard the circumstance as a good omen, to be “born with a caul.” Those vertebrates which possess an amnion are termed Amnionata; those which do not, Anamnionata: terms coincident respectively with Allantoidea and Anallantoidea.
2. n. In entomology, a membrane which surrounds the larva of many insects, as the millepeds (Iulidæ), for some time after they are batched from the egg. It is regarded by some as the analogue of the amnion of a vertebrate.
3. n. A reflected portion of a membrane, in ascidians, which lines the inner wall of the ovisac, and forms a kind of amniotic investment of the embryo.
4. n. In botany, a name formerly given to the fluid contents of the embryo sac.
5. n. Sometimes erroneously written amnios.
6. n. In echinoderms, the sac in the pluteus larva inclosing the developing echinus.

Wiktionary

1. n. anatomy the innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended.

GNU Webster's 1913

1. n. (Anat.) A thin membrane surrounding the embryos of mammals, birds, and reptiles.

WordNet 3.0

1. n. thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)