We have seen that the intestinal tube of all vertebrates is the product of two of the embryonal blastodermic layers, the entoderm and mesoderm. The former furnishes the characteristic and cardinal elements of the digestive tract, viz., the secretory and absorbing epithelium of the mucous membrane and of the accessory digestive glands, the liver and pancreas. From the mesoderm, on the other hand, are derived the muscular and connective tissue coats which surround the epithelial tube and contribute to the thickness of the intestinal wall, as well as the blood vessels and lymphatics. The alimentary canal separates from the yolk-sac of the embryo by the development of cranial, caudal and lateral folds, and at an early period communicates with the neural canal by the primitive postanal gut (cf. p. 23). This connection subsequently becomes lost. The oral and anal openings, by means of which the alimentary canal communicates with the exterior, are formed _secondarily_ by entodermal invaginations which finally break through into the lumen of the canal (cf. p. 24). At an early embryonic stage the alimentary canal appears therefore as a straight cylindrical tube running cephalo-caudad in the long axis of the body-cavity, and suspended by the primitive mesentery from the ventral aspect of the chorda dorsalis. In Amphioxus, the cyclostomata, certain teleosts, dipnoeans and lower amphibians the canal remains permanently in this condition (cf. Fig. 310). In the remaining vertebrates the uniform non-differentiated tube of the embryo develops further and appears more or less distinctly divided into a proximal segment, the _foregut_, a central segment, the _midgut_, and a distal segment, the _hindgut_, or _endgut_. This differentiation of the tube into successive segments is closely connected with the character and quantity of the food habitually taken and with the method and rapidity of its elaboration in the process of digestion, absorption and excretion. In general the _foregut_ is formed by the segment which succeeds to the oral cavity, and includes the _pharynx_, _oesophagus_ and _stomach_. The _midgut_ is composed of a longer or shorter narrower tube of nearly uniform caliber, the _small intestine_, which follows the gastric dilatation. Even in forms in which the stomach is not distinctly differentiated (cf. p. 40) the connection of the biliary duct with the intestinal canal serves to separate the fore- and midgut. The _hindgut_ or _large intestine_ is usually separated from the preceding segment by an external circular constriction, with a corresponding annular valve or fold of the mucous membrane in the interior. The beginning of the large intestine is marked in many forms by the development of an accessory pouch or diverticulum, the _caecum_. The hindgut extends from its junction with the midgut to the cloacal or anal opening. 1. Midgut or Small Intestine.