While the obliteration and complete absorption of the duct is normal in nearly all vertebrates, a remnant persists in some birds, in which a short caecal pouch (_diverticulum caecum vitelli_) is found at about the middle of the small intestine. A portion of the vitello-intestinal duct thus persists throughout life in some wading and swimming birds. Figs. 39 and 40 show this condition in the small intestine of _Urinator lumme_ and _imber_, the red-throated loon and the great northern diver. In other birds, however, such as birds of prey, song birds, etc., the duct is absorbed and disappears completely. [Illustration: FIG. 39.--Small intestine of the red-throated loon, _Urinator lumme_, showing persistent caecal pouch, the remnant of the vitelline duct. (Columbia University Museum, No. 997.)] [Illustration: FIG. 40.--Small intestine of great northern diver, _Urinator imber_, with caecal pouch, the remnant of the vitelline duct. (Columbia University Museum, No. 77, 1578.)] In order to complete the embryological history of the alimentary canal it is necessary to take brief account of another structure derived from it, namely the _allantois_. Its significance to the adult organism is seen in connection with the genito-urinary tract, the urinary bladder being formed by its persistent portion. In the embryo, however, it has important nutritive and respiratory functions. In the embryos of the higher vertebrates nutrition depends only in the earliest stages upon the yolk-sac of the ovum, over which a vascular network extends. Very soon the caudal portion of the primitive intestine develops a vascular sac-like outgrowth (Figs. 21 and 41). This pouch forms the _allantois_. It is intimately connected with embryonal respiration, and probably also forms a reservoir which receives the secretion of the primitive kidney. This foreshadows the final destiny of the proximal intra-abdominal portion of the allantoic sac which persists and is converted into the urinary bladder of the adult. [Illustration: FIG. 41.--Diagram illustrating the later stages in the formation of the mammalian foetal membranes. (Heisler, modified from Roule.)]