The digestive apparatus includes the actual intestinal tract, taking those words in their widest sense, and according to its development this tract may be divided into four portions: The oral cavity; the foregut; the midgut; the hindgut.
The oral cavity extends from the lips to the isthmus of the fauces; the foregut comprises the pharynx, the oesophagus, and the stomach; the midgut is identical with the small intestine; and the hindgut is composed of the large intestine and the rectum. The tract commences at the mouth and terminates at the anus.
Associated with the digestive tube are a large number of glandular appendages, namely, the small and large salivary glands in the mouth; the pharyngeal, oesophageal, and gastric glands in the foregut; and the duodenal and intestinal glands, as well as the two largest glands of the digestive apparatus, the pancreas and the liver, in the midgut. The hindgut has only the intestinal glands situated in its walls. Although the spleen is not really an organ of the digestive tract, since it originates in the mesenchyma and not from the entoderm, it is usually described with the digestive apparatus. The wall of the digestive tube also contains lymphatic aggregations of variable size, whose chief peculiarity is that their parenchyma is adherent to the superficial epithelium of the gut, which thus becomes infiltrated by their cellular elements. In the upper portion of the tract the larger of these aggregations are designated as tonsils, in the lower portion as aggregated lymphatic follicles (Peyer's patches).
For a certain period in the human embryo the gut is a completely closed straight tube possessing neither an oral nor an anal opening. These orifices are formed later, when the tract has undergone further differentiation, as oral and anal ectodermic depressions which deepen and gradually approach the anterior and posterior portions of the intestinal tube, until the lumen of the tube is separated from the outer world only by thin membranes, known respectively as the pharyngeal and anal membranes. With the rupture of these membranes the two primary body openings are formed, from which are soon developed by the formation of septa the oral and nasal cavities upon the one hand and the anal and urogenital orifices upon the other. The oral depression forms a considerable portion of the subsequent oral cavity, which is consequently very largely of ectodermic rather than of entodermic origin.
From Human Anatomy (1909) by DR. Johannes Sobotta (1869-1945) Professor of Anatomy in the university of Wurzburg