A distinct digestive segment may even be entirely wanting, owing to its failure to differentiate from the oesophagus on the one hand and from the endgut on the other. In such forms the entire digestive canal appears as a tube of uniform caliber extending from mouth to anus. It is necessary to begin with these simple structural conditions in order to obtain a clear conception of the disposition of the viscera in the adult human abdomen. Such simple arrangement of the alimentary tract is found in the embryo of man and of the higher vertebrates, and similar rudimentary types are encountered, as the permanent condition, in some of the lower forms. These latter are especially valuable for purposes of study, because they afford an opportunity of examining directly, as macroscopic objects, structural conditions which are found only as temporary embryonal stages during the development of the higher mammalia (Fig. 43). In the early stages the alimentary tract of the mammalian embryo consists of a straight tube of nearly uniform caliber (Fig. 44, _A_), extending from the pharynx to the cloaca, along the median line in the dorsal region of the body cavity, connected with the ventral aspect of the axial mesoderm by a membranous fold forming the primitive common dorsal mesentery. Subsequently differentiation of this simple tube into successive segments takes place, marked by differences in shape and caliber and in histological structure. [Illustration: FIG. 44.--Schematic diagram representing three stages in the differentiation of the mammalian digestive tract: A. Early undifferentiated stage, in which the entire canal appears as a tube of uniform calibre. B. Spindle-shaped gastric dilatation. C. Typical mammalian gastric dilatation.] [Illustration: FIG. 45.--Reconstruction of human embryo. 1, 2, 3, 4, Gill-pouches. (After Fol.)] The first indication of the future stomach appears early, in human embryos of from 5-6 days (Figs. 44, _B_, and 45; for later embryonal stomach forms compare also Figs. 33, 35 and 36), as a small spindle-shaped dilatation of a portion of the primitive entodermal tube, placed in the median plane, dorsad of the embryonic outgrowth of the liver, between it and the oesophagus. The appearance of this dilatation marks the separation of the proximal cephalic part (pharynx and oesophagus) from the distal caudal (intestinal) portion of the primitive alimentary canal. Further growth of the stomach takes place chiefly along the dorsal margin of the dilatation, rendering the same more convex. The ventral border develops to a less degree and in the course of further and more complete differentiation the dorsal margin of the future stomach assumes even at this period the character of the greater curvature, while the opposite ventral margin, the future lesser curvature, following the dilatation of the tube dorsad, becomes in turn concave (Fig. 44, _C_).