[Illustration: FIG. 39.--Small intestine of the red-throated loon, _Urinator lumme_, showing persistent cæcal pouch, the remnant of the vitelline duct. (Columbia University Museum, No. 997.)]
[Illustration: FIG. 40.--Small intestine of great northern diver, _Urinator imber_, with cæcal 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 fœtal membranes. (Heisler, modified from Roule.)]
The allantois is present in Amphibia but is very small. In Amniota[1] it is large and grows around the embryo. In those of the higher vertebrates which are developed within an egg (reptiles, birds and monotremes) the sac of the allantois comes to lie beneath the egg-shell and acts as a respiratory organ. In the higher mammalia, developed within the uterus, the allantois becomes attached by vascular villi to the uterine wall and establishes a vascular connection between the fœtal and maternal blood vessels. In this way the _allantoic placenta_ is formed (Fig. 41). The placenta, as just stated, is absent in the monotremes and is only slightly developed in marsupials, in which animals the fœtus develops to maturity in the marsupial pouch after leaving the uterus. These animals are therefore distinguished as _Aplacentalia_ from the remaining higher mammals in which the allantoic placenta develops and which are hence called the _Placentalia_.