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Related to allantois: yolk sac, urachus, Vitelline duct


A fluid-filled sac- or sausagelike, extraembryonic membrane lying between the outer chorion and the inner amnion and yolk sac of the embryos of reptiles, birds, and mammals. It is composed of an inner layer of endoderm cells, continuous with the endoderm of the embryonic gut, or digestive tract, and an outer layer of mesoderm, continuous with the splanchnic mesoderm of the embryo. It arises as an outpouching of the ventral floor of the hindgut and dilates into a large allantoic sac which spreads throughout the extraembryonic coelom. The allantois remains connected to the hindgut by a narrower allantoic stalk which runs through the umbilical cord. See Amnion, Chorion, Germ layers

The allantois eventually fuses with the overlying chorion to form the compound chorioallantois, which lies just below the shell membranes in reptiles and birds. The chorioallantois is supplied with an extensive network of blood vessels and serves as an important respiratory and excretory organ for gaseous interchange. The allantoic cavity also serves as a reservoir for kidney wastes in some mammals, in reptiles, and in birds. In the latter two groups the allantois assists in the absorption of albumin. In some mammals, including humans, the allantois is vestigial and may regress, yet the homologous blood vessels persist as the important umbilical arteries and veins connecting the embryo with the placenta. See Fetal membrane, Placentation

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


A fluid-filled, saclike, extraembryonic membrane lying between the chorion and amnion of reptilian, bird, and mammalian embryos.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Ownership of UCB is sometimes debatable since the umbilical cord is embryologically derived from the fetal allantois; it may be considered property of the child.
The intra-embryonic cloaca, destined to become the bladder, is connected to the extra-embryonic allantois derived from the caudal end of the yolk sac through the urachus.
Underdeveloped trophoblast, precocious regression of the yolk sac and/or a failure of the allantois and vasculature to develop may delay attachment and placentation and, if threshold for survival is not attained, such events will likely be lethal.
The chorioallantoic membrane develops from two extraembryonic structures: the chorion and allantois. The vascular system of the CAM consists of the capillary network close to the chorion epithelium as well as larger and deep free-floating vessels that move with movements of the embryo.
The placentas of eutherian mammals consist of two extra-embryonic membranes; that is, the allantois and the chorion.
Stage 1 ends when the allantois, or fetal membranes, are pushed through the cervix by the advancing fetus and rupture, releasing amniotic fluid (breaking water).
Blood Vessels Number, Endothelial Vascular Growth Factor, Induced Nitric Oxide Synthase and Endothelial Nitric Oxide Synthase Expression in the Ovine Allantois
As the allantois develops, the yolk sac becomes a shriveled vestige.
The urachus is a vestigial fibrous remnant of the allantois, the canal that drains urine from the developing fetus.
Before birth, urinary bladder communicates with allantois through urachus which becomes atrophied and its lumen gets obliterated after parturition (Laverty and Salisbury, 2002).
The urachus is an embryological remnant of the allantois. Glandular tumours are the most common neoplasms arising from urachal remnants and they exist on a spectrum from benign mucinous cystic tumours to malignant noncystic neoplasms [1].
Urachus or median umbilical ligament is obliterated allantois. It gets obliterated in fetal life.