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 foetal 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 foetus 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_. [1] In the embryos of reptiles, birds and mammals folds of the somatopleure arise externally to the constricting furrows by means of which the embryo is gradually separated from the yolk-sac, with the resulting formation of the intestinal and abdominal walls. These folds, situated at the head, tail and on the sides, grow upwards and finally meet and unite to form a membranous sac called the _amnion_. Hence these higher vertebrates (reptiles, birds and mammals) are called _Amniota_, in contradistinction to fishes and amphibia who have no amnion and are hence known as _Anamnia_. =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,} The liver, } Connected by their ducts with the digestive The pancreas, } canal. 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.