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.
Summary.—To recapitulate, therefore, the intestinal tube gives origin to two kinds of appendages or derivatives:
1. Organs of the adult body, derived by budding from the alimentary entodermal epithelium, in the form of pouch-like diverticula which follow the glandular type of development and become secondarily associated with mesodermal elements. These organs are again of two kinds:
(a) Organs which retain their original connection with the lumen of the digestive canal:
| The salivary glands, |  | Connected by their ducts with the digestive canal. |
| The liver, | ||
| The pancreas, | ||
| The lungs, | ||
| which open by means of the trachea and the laryngeal aperture into the pharyngeal cavum. | ||
(b) Organs which lose their primitive connection with the alimentary canal.
Thymus and Thyroid Gland.
2. Embryonic appendages of the alimentary tract.
(a) The vitello-intestinal or omphalo-mesenteric duct and the yolk-sac or umbilical vesicle. This structure does not form as an extension from the intestinal tube after the same has been closed by coalescence of the splanchnopleure in the ventral mid-line, but is the result of the folding in of the layers of the embryonic germinal area, by means of which the body-rudiment is constricted off from the yolk-sac. The reduced channel of communication forms the vitello-intestinal duct. In the vast majority of vertebrates this disappears completely by absorption in the course of further development. It may persist in part abnormally as Meckel’s diverticulum. In a few birds its proximal portion remains normally as a small blind pouch attached to the free border of the small intestine.