Dua, (from the Latin, de,from, and cado, to fall). Falling, fading once in the year; whatever falls away, as leaves of trees. In botany, deciduous plants are such as cast their leaves in winter. From this Dr. Hunter calls the spongy chorion by the names decidua and caduca. (See Abortus.) Dr. Hamilton observes that the membranes (speaking of those which contain the foetus during pregnancy) consist, externally, of two layers of the spongy chorion, called decidua and decidua reflexa: internally,of the true chorion, and the amnion. They form a pretty strong bag, commencing at the edge of the cake, going round the whole circumference, and lining the internal surface of the womb. The membrana decidua, or that lamella of the spongy false chorion which is in immediate contact with the uterus, is originally very thick and spongy, and exceedingly vascular, particularly where it approaches the placenta. At first it is loosely spread over the ovum; and the intervening space filled with a quantity of gelatinous substance; but gradually becomes more and more attenuated, and approaches nearer to the interior lamella of the decidua, called decidua reflexa: about the fifth month the two layers come in contact, and adhere so as to become apparently one membrane. The decidua reflexa, in its structure and appearance, is similar to the former, being rough, fleecy, and vascular, on its external surface; internally, smoother, and perforated with a number of small foramina, which are the orifices of vessels that open on this internal surface. In advanced gestation it adheres intimately to the former membrane, and is with difficulty separated when the double decidua comes off entire; but the outer lamella more commonly adheres to the uterus after the placenta and other membranes are expelled, and is afterwards cast off with the cleansings. The decidua reflexa also becomes thicker and more vascular as it approaches the placenta; and is then blended with its substance, constituting the cellular or maternal part of the cake. The other, or more internal part, belongs to the foetus, and is styled the fetal part of the placenta. The double decidua is opaque, in comparison of the other membranes; the blood vessels are derived from the uterus, and can be readily traced into it. Dr. Hunter supposes that the double decidua lines the uterus nearly in the same manner as the peritonaeum does the cavity of the abdomen, and that the ovum is inclosed within its dupli-cature, as within a double night cap. On this supposition, the ovum must be placed on the outside of this membrane; which is not very easily comprehended, unless we adopt Signor Scarpa's opinion, and suppose it to be originally composed of an inspissated coagulable lymph. Dr. Ruysch called this exterior coat the tunica fi/a-mentosa; more modern authors, the false or spongy chorion, consisting, as we have said, of two distinct layers. The portion which covers the ovum is a complete membrane, like the true chorion and amnios; but that which immediately lines the uterus is imperfect or deficient, being perforated with three foramina, viz. two small ones, corresponding with the insertion of the tubes at the fundus uteri; and a larger ragged perforation opposite to the orifice of the womb. See Dr. Hamilton's Outlines; Dr. Hunter on the Gravid Uterus.