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, dipnœans 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, œsophagus 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 cæcum. The hindgut extends from its junction with the midgut to the cloacal or anal opening.
1. Midgut or Small Intestine.
The small intestine is the segment of the alimentary canal in which digestion of the non-nitrogenous food substances takes place, and which affords the necessary area of mucous surface for the absorption of all digested matters. Consequently the character and habitual quantity of the food here elaborated exerts a very marked influence on the length of the small intestine, i. e., on the extent of the digestive and absorbing surface represented by its mucous membrane.
The relative length of the small intestine in any individual form will vary with both the quantity and volume of the food and with the rapidity of the metabolic processes. Animals, in which digestion is rapid and the usual food small in bulk and concentrated in its nutrient qualities, have a relatively short intestine, while the canal is longer in forms subsisting on food which is bulky and which demands considerable time for its elaboration. Hence we find the relatively shortest intestine in carnivora, the longest in herbivora, while the canal in omnivora occupies an intermediate position in regard to its relative length.
The rapidity of tissue-metabolism also exerts a marked influence on the length and development of this portion of the alimentary canal.