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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


A fluid-filled, saclike, extraembryonic membrane lying between the chorion and amnion of reptilian, bird, and mammalian embryos.
References in periodicals archive ?
In conclusion, in order to have a successful vascular development in the allantois it appears that VEGF, iNOS and eNOS must be expressed which could provoke liberation of nitric oxide thus enhancing the angiogenic factor function, to favour embryonic viability.
As the allantois develops, the yolk sac becomes a shriveled vestige.
A partial or total defect in the process of obliteration of the allantois results in urachal abnormalities.
Virus infectious titer in the lungs of infected mice and allantois liquid was determined by contamination 9 -10 days chicken embryos and expressed it in log [EID.
Hydrallantois or hydrops of allantois is a pathological condition affecting pregnant uterus characterized by rapid accumulation of watery, amber colored fluid inside allantoic cavity over a period of 5-20 days in late gestation and is always giving suspicion for twin/triplet pregnancy (Morrow, 1986).
In early fetal development, the ventral cloaca (precursor of the urinary bladder) is connected to the allantois via the urachus, which undergoes progressive involution, resulting in an obliterated, fibrous cord by 32 weeks' gestation.
The mesoderm also contributes to formation of the amnion and allantois.
Ownership of umbilical cord blood is sometimes debatable; since the umbilical cord is embryologically derived from the fetal allantois, it may be considered property of the child.
At this stage, an allantois is developing from the hindgut.
In the specimen examined presented vestige of allantois sack and vitelline sack in these gestational phase.
Hence, to meet these nutritional requirements, fetal membranes were evolved namely amnion, chorion, yolk sac, allantois, placenta and umbilical cord.
Indications for caesarean section include foetal oversize, feto-pelvic disproportion, incomplete dilation of cervix, irreducible torsion of uterus, hydrops of foetus, hydrops allantois or abnormalities of presentation, position or posture, ventral hernias and prolonged cases of dystocia (Roberts, 1996).