allantois

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A membranous sac that develops from the posterior part of the alimentary canal in the embryos of mammals, birds, and reptiles. It is important in the formation of the umbilical cord and placenta in mammals. Also called allantoid.

Century Dictionary and Cyclopedia

1. n. A fetal appendage of most vertebrates, developing as a sac or diverticulum from the posterior portion of the intestinal cavity. It is one of the organs of the embryo of all amniotic vertebrates, or those which develop an amnion, but is wanting or is at most rudimentary in amphibians and fishes. In birds and reptiles it is large and performs a respiratory function, and in mammals contributes to form the umbilical cord and placenta. Its exterior primitively consists of mesoblast, its cavity receiving the secretion of the primordial kidneys (Wolffian bodies). So much of the sac as remains pervious within the body of the embryo becomes the urinary bladder, or, in some degree, a urinary passage. The umbilical arteries and veins course along the elongated stalk of the sac, which becomes the umbilical cord, and that part of these allantoic vessels within the body which does not remain pervious becomes the urachus and round ligament of the liver. The expanded extremity of the allantois, in most mammals, unites with the chorion to form the placenta. In those vertebrates, as mammals, in which the umbilical vesicle has but a brief period of activity, the allantois chiefly sustains the functions whereby the fetus is nourished by the blood of the mother, and has its own blood arterialized. In parturition, so much of the allantois as is outside the body of the fetus is cast off, the separation taking place at the navel. See cut under amnion.

Wiktionary

1. n. anatomy A sac, having a number of functions, that develops in the alimentary canal of the embryos of mammals, birds and reptiles.

GNU Webster's 1913

1. n. (Anat.) A membranous appendage of the embryos of mammals, birds, and reptiles, -- in mammals serving to connect the fetus with the parent; the urinary vesicle.

WordNet 3.0

1. n. the vascular fetal membrane that lies below the chorion and develops from the hindgut in many embryonic higher vertebrates (reptiles, birds and mammals)