Allantois, one of the foetal membranes present in the embryos of reptiles, birds, and mammals. It commences as an outgrowth from the hinder portion of the intestinal canal, which gradually enlarges, insinuating itself with its vessels between the amniotic folds, until it comes into contact with the shell membrane. In birds the allantois undergoes considerable development and serves as an aerating organ to the growing embryo, which it completely envelops. In man and mammals the allantoic vessels are only distributed over part of the outer membrane, that part, namely, where the placenta (q.v.) will be formed. The internal part of the allantois persists in man as the urachus (q.v.). Of the five groups of vertebrate animals, two, namely, fish and amphibia, have no allantois.