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So called on account of its being considered to be discharged from the uterus at parturition. A membrane, formerly defined to be the outermost membrane of the foetus in utero; and still so defined by some- as by Dr. Lee. Chaussier calls it Epichorion: by others, it has been called Membrana caduca Hunteri, M. flocculeuta seu cellulosa seu sinuosa seu communis seu proeexistens, Decidua externa seu spongiosa, Tunica exterior ovi seu caduca seu crassa, Membrana cribrosa seu ovi materna seu mucosa seu uteri interna evoluta, Epione, Placenta uteri succenturiatus, Subplacenta, Nidamentum, Anhistous membrane, (F.) Caduque, C. vraie, Membrane caduque, Epione, Perione; -prior to the time of the Hunters, called Chorion spongiosum seu tomentosum seu fungosum seu reticulatum, &c., Tunicafilamentosa, Shaggy Chorion, Spongy Chorion, &c. Great diversity has prevailed regarding this membrane and its reflected portion. It exists before the germ arrives in the uterus- as it has been met with in tubal and ovarial pregnancies; and is occasioned by a new action, assumed by the uterine vessels at the moment of conception. Chaussier, Lobstein, Gardien, Velpeau, and others, consider it to be a sac, without apertures, completely lining the uterus, and that when the ovum descends through the tube, it pushes the decidua before it, and becomes enveloped in it, except at the part destined to form the placenta. That portion of the membrane which covers the ovum, forms the Membrana decidua reflexa, (F.) Caduque reflechie, Membrane caduque reflechie, according to them -the part lining the uterus being the Decidua Uteri, (F.) Caduque uterine. Towards the end of the fourth month, the decidua reflexa disappears. The very existence of a Tunica decidua reflexa has, however, been denied. This last membrane has received various names. Dr. Granville, regarding it as the external membrane of the ovum, has termed it Cortex ovi. It has also been termed Involucrum membranaceum, Membrana retiformis chorii seu filamentosa seu adventitia seu crassa, Ovuline and Decidua protrusa. To the membrane which, according to Bojanus and others, is situate between the placenta and the Uterus, and which he considers to be produced at a later period than the decidua vera, he gave the name membrana decidua serotina. Histological researches seem to show, that the decidua is an altered condition of the lining membrane of the uterus, with a whitish secretion filling the uterine tubular glands. The decidua reflexa is probably formed by the agency of nucleated cells from the plastic materials thrown out from the decidua uteri; in the same manner as the chorion is formed in the Fallopian Tube from plastic materials thrown out from its lining membrane. That the decidua reflexa is not a mere inverted portion of the decidua uteri is shown by the fact, that the texture of the two is by no means identical.