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Chicken egg in its ninth day of development

Allantois (Gr. Ho allás , allántos "the sausage-shaped") is the name given to the embryonic urinary bladder in reptiles , birds and mammals . It is a protuberance of the embryonic rectum and is used, among other things, to store the water of oxidation and other metabolic end products. The allantois is often permeated with blood vessels .

Humans and other mammals

In humans, the allantois arises from the hypoblast on the 16th day after conception and is normally only present temporarily. It initially extends as an allantoic diverticulum into the extraembryonic mesoderm of the body stalk - the later umbilical cord of the embryo. The allantoic diverticulum hardly enlarges in the human embryo and closes to form the urachus . If the allantois does not completely regress in humans, an urachus fistula can remain, which creates a complete connection between the bladder and the umbilicus.

In most other mammals, the allantois diverticulum is surrounded by a large sac of mesodermal origin, the allantois, which lies in the chorionic cavity and surrounds the entire amniotic cavity (e.g. predators , horses ) or only part of it (e.g. artifacts ) . The allantois forms the placenta through its connection with the chorion ( allantochorion ) .


In the domestic fowl, the allantois is about as large as the embryo after five days of incubation and has already pierced the amnion . It then grows in the egg towards the air chamber and comes into contact with it. Thereafter, gas exchange between the embryo and the area around the egg takes place to a considerable extent via the allantois. The growth continues and the allantois touches almost the entire surface of the egg inside just before the chick hatches. At this stage, the allantois also helps break down the calcium from the shell, which is necessary for building bones . This process is mediated by the exhaled carbon dioxide and the moisture in the shell. When hatching, the allantois is destroyed and the remaining content wets the chick together with the remaining fluid from the amnion ( amniotic fluid ).


The conditions in reptiles are similar to those in birds. In some species, such as the viviparous sea ​​snakes of the genus Enhydrina , the allantois fulfills the function of a placenta (allantoid placenta).


  • P. Fioroni: General and Comparative Embryology of Animals. Springer, Berlin 1992.
  • T. Schiebler et al. (Ed.): Anatomi. 6th edition. Springer, Berlin, 1995.
  • Detlev Drenckhahn (Ed.): Benninghoff, Anatomie . Volume 1, 17th edition. Urban & Fischer-Verlag, 2008, ISBN 978-3-437-42342-0 , p. 228.
  • Monika Kressin, Bertram Schnorr: Embryology of Pets. 5th edition. Enke-Verlag, Stuttgart 2006, ISBN 3-8304-1061-1 .