In its earlier stages the primitive common mesentery forms a membrane which carries the intestinal blood vessels between its two layers, surrounds the embryonic alimentary canal and attaches the same to the ventral aspect of the chorda dorsalis and aorta. This is the permanent condition in many of the lower vertebrates in which the intestinal tube is suspended by a simple dorsal mesentery, a condition which is repeated by the embryos of man and the higher vertebrates. From this primitive common mesentery are derived, by further development, displacement and adhesion, all the other mesenteries, omenta and peritoneal folds of the adult. The character and degree of these subsequent changes is determined by the increase in length and change in position of the intestine and the growth of large organs, like liver, spleen and pancreas. Many portions of the intestinal canal, at first suspended by the mesentery and freely movable within the abdominal cavity, become later, by secondary adhesion, firmly connected with adjacent portions of the tube or with the abdominal parietes. In certain of the lower vertebrates (fishes) large sections of the intestine lie entirely free within the abdomen, their only connection with the parietes being afforded by the blood vessels. This condition depends upon _absorption_ of the original mesentery. A similar process, though much more circumscribed, is observed in the omenta of many mammals, which appear perforated at several points. =Derivatives of the Entodermal Intestinal Tube.=--The entodermal epithelium is physiologically the characteristic element of the alimentary canal. Besides lining the entire internal surface of the tube it gives rise by budding and protrusion from the intestinal canal to a series of organs which from the mode of their development must be regarded as diverticular or derivatives of the alimentary canal (Figs. 33, 34, and 35). These organs, proceeding in order cephalo-caudad, are the following: The salivary glands. Thymus and thyroid. The lungs. Pancreas. Liver. [Illustration: FIG. 33.--Schema of alimentary canal and accessory organs, derived from same. (After Bonnet.)] [Illustration: FIG. 34.--Reconstruction of alimentary canal of human embryo of 4.2 mm. x 24. (After His.)] [Illustration: FIG. 35.--Reconstruction of alimentary canal of human embryo of 7 mm. (twenty-eight days). x 12. (After His.)]