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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 ?
It has been reported that there are two pathways for the degradation of allantoic acid in soybean leaves.
The infective allantoic fluid was then inactivated with appropriate inactivating agent (0.
Chicken embryo lesions: The 10-day-old SPF chicken embryo eggs were inoculated 200 l allantoic fluid of 103 EID50, the eggs were observed for embryo mortality and lesions.
The samples were propagated in specific pathogen-free embryonated chicken eggs, and the allantoic fluid was subjected to qRT-PCR.
Allantoic fluid from the injected eggs agglutinated 0.
The influenza A (H7N9) genome that we report varies from that obtained by Sanger sequencing after passage in the allantoic sac and amniotic cavity of 9-11-day-old specific pathogen-free embryonated chicken eggs for 48-72 hours at 35[degrees]C (Table 1).
Viruses were isolated from RT-PCR-positive clinical samples collected from each eye by inoculating embryonated chicken eggs and incubating them for 48 h before harvest of allantoic fluid.
Also, embryonated eggs were inoculated by the allantoic route with swab samples collected during 2004-2005.
We attempted to infect Pekin duck embryos through the allantoic cavity, as well as by intravenous infection routes, and were not able to demonstrate replication (data not shown).