What does allantois mean?
The allantois is one of the four extraembryonic membranes found in the embryos of reptiles, birds, and mammals — a group known as amniotes. It develops as an outgrowth from the hindgut, the rear portion of the embryonic digestive tract. Its roles vary by group: in birds and reptiles it expands beneath the shell, where it takes up oxygen, releases carbon dioxide, and stores nitrogenous waste; in placental mammals its blood vessels grow outward to become those of the umbilical cord, and the allantois itself contributes to the formation of the placenta. In humans it is small but significant, later giving rise to parts of the urinary bladder. Because it is central to reproduction in land vertebrates, the word belongs chiefly to anatomy, embryology, and veterinary medicine, appearing most often in textbooks and research literature rather than everyday speech. Its derived adjective 'allantoic' is common in phrases such as allantoic fluid and allantoic circulation.
nounA fetal membrane that develops as an outpouching of the hindgut in reptiles, birds, and mammals, and which in humans contributes blood vessels to the umbilical cord and becomes part of the bladder.
- Anatomy/Embryology: an extraembryonic membrane arising from the posterior part of the gut of amniote embryos; in mammals it helps form the placenta and umbilical vessels, and in birds and reptiles it functions in respiration and waste storage.
"In the chick embryo, the allantois expands until it lies just beneath the shell, where it serves as a respiratory organ."
"Blood vessels extending from the allantois ultimately become the arteries and veins of the umbilical cord."
"Because the allantois stores nitrogenous waste in the developing egg, it was a crucial innovation in the evolution of land-dwelling vertebrates."
Rarely needed; because the term refers to a single structure within each embryo, plural use occurs only in comparative or pathological contexts.
"Comparisons across species revealed striking differences in the vascularization of their allantoises."
Before your placenta existed, a tiny sac growing from what would become your bladder helped build it — and its hollow remnant survives in adults as a ligament running from navel to bladder.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of allantois
The word allantois comes from New Latin, borrowed from Greek 'allantoeidēs', meaning 'sausage-shaped', from 'allas' (genitive 'allantos'), meaning sausage. It entered English scientific vocabulary around the mid-seventeenth century as anatomists described the tubular, sausage-like appearance of this fetal membrane in dissections. Related forms include the adjectives allantoic and allantoid, both built on the same Greek root.
Related word forms
How allantois is actually used
Allantois is a technical term confined almost entirely to anatomy, embryology, and veterinary science; it appears rarely outside scientific writing. It is usually discussed alongside the other extraembryonic membranes (the yolk sac, amnion, and chorion). The adjective allantoic is far more common in practice, especially in phrases such as 'allantoic fluid' in descriptions of bird eggs or livestock pregnancy.
Easily confused with allantois
The amnion is the innermost membrane surrounding the embryo in a fluid-filled sac, whereas the allantois is an outer-lying extraembryonic membrane involved in waste storage, gas exchange, and umbilical vessel formation.
The chorion is the outermost embryonic membrane that contacts the shell or uterine wall, while the allantois arises from the hindgut and in many species fuses with the chorion to form the chorioallantoic membrane.