Fig. 35.—Reconstruction of alimentary canal of human embryo of 7 mm. (twenty-eight days). × 12. (After His.)   Fig. 36.—Reconstruction of alimentary canal of human embryo of thirty-five days (13.8 mm.). × 8. (After His.)

The portion situated caudad of the duct produces the rest of the small and all of the large intestine (Figs. 33 and 35). At times in man and other mammals (cat) the vitello-intestinal duct does not become absorbed, but persists and continues to develop as a part of the small intestine, forming the blind pouch or appendage known as Meckel’s diverticulum (Figs. 37 and 38). This diverticulum may vary in length from 1.5 to 15 cm. It either projects freely into the abdominal cavity as a pouch arising from the convex border of the small intestine opposite to the mesenteric attachment, or else it reaches the abdominal wall at the umbilicus and is attached to the same. In a few instances it has not terminated in a blind pouch, but has remained open at the umbilicus, in which case the aperture discharges intestinal contents. Sometimes the process of obliteration which normally leads to the absorption of the vitello-intestinal duct extends to the adjoining segment of the small intestine, resulting in obliteration of the intestinal lumen and consequent obstruction at this point.

Fig. 37.—Human adult ileum with Meckel’s diverticulum. Ileo-diverticular serous fold and persistent omphalo-mesenteric artery. (Columbia University Museum, No. 1803.)   Fig. 38.—Human adult ileum, with Meckel’s diverticulum. (Columbia University Museum, No. 745.)

The intestinal opening of the diverticulum is situated at a varying distance above the ileo-colic junction, ranging from 27.5 cm. to 290 cm., with an average of 107 cm.

While the obliteration and complete absorption of the duct is normal in nearly all vertebrates, a remnant persists in some birds, in which a short cæcal pouch (diverticulum cæcum vitelli) is found at about the middle of the small intestine. A portion of the vitello-intestinal duct thus persists throughout life in some wading and swimming birds. Figs. 39 and 40 show this condition in the small intestine of Urinator lumme and imber, the red-throated loon and the great northern diver. In other birds, however, such as birds of prey, song birds, etc., the duct is absorbed and disappears completely.

Fig. 39.—Small intestine of the red-throated loon, Urinator lumme, showing persistent cæcal pouch, the remnant of the vitelline duct. (Columbia University Museum, No. 997.)   Fig. 40.—Small intestine of great northern diver, Urinator imber, with cæcal pouch, the remnant of the vitelline duct. (Columbia University Museum, No. 77, 1578.)

In order to complete the embryological history of the alimentary canal it is necessary to take brief account of another structure derived from it, namely the allantois. Its significance to the adult organism is seen in connection with the genito-urinary tract, the urinary bladder being formed by its persistent portion. In the embryo, however, it has important nutritive and respiratory functions. In the embryos of the higher vertebrates nutrition depends only in the earliest stages upon the yolk-sac of the ovum, over which a vascular network extends.

Fig. 41.—Diagram illustrating the later stages in the formation of the mammalian fœtal membranes. (Heisler, modified from Roule.)

Very soon the caudal portion of the primitive intestine develops a vascular sac-like outgrowth ([Figs. 21] and 41). This pouch forms the allantois. It is intimately connected with embryonal respiration, and probably also forms a reservoir which receives the secretion of the primitive kidney. This foreshadows the final destiny of the proximal intra-abdominal portion of the allantoic sac which persists and is converted into the urinary bladder of the adult.