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Show ^°^ DR. P. CHALMERS MITCHELL ON [Dec. 1, 4. On the Occasional Transformation of Meckel's Diverticulum in Birds into a Gland. By P. C H A L M E RS M I T C H E L L , M.A., D Be., Secretary to the Society. [Received December 1, 1903.] (Text-figure 44.) Soon after 1 began to study the varying dispositions of the intestines in Birds I, like other observers, was struck by the occasional presence of a well-marked caecum about the middle of the course of the small intestine. This caecum is identical morphologically with what is known as Meckel's diverticulum in the case of Man, and is the vestige of the embryonic yolk-sac. In a communication (Trans. Linn. Soc. ser. 2, Zool. vol. viii. pt. 7, pp. 173-275, 1901) in which I reviewed the structure of the intestinal tract in many hundreds of birds belonging to practically all the families, I was able to add considerably to our knowledge of the occurrence of this structure and to point out its morphological significance as a point of orientation. In mammals, its presence in the adult appears to be rare and to be only an individual variation. In birds, the curious state of the case is that its presence or absence appears not to be an individual variation but to mark coherent assemblages. In most cases, when present, it is a true vestige of the embryonic condition, an apparently functionless rudiment sometimes still containing small granules of yolk. In such cases it appears to get smaller with age, but on this point I have not a large collection of observations to guide me. In other cases, a more peculiar condition occurs-the yolk-sac vestige has, so to speak, burst out again into a second life. It has become a glandular organ of a highly elaborate nature. I have already stated this fact {loc. cit. p. 264) and mentioned that Lonnberg and Jagerskiold have drawn attention to it; but as the microscopic structure of this new organ has not been described, I propose here to give a short account of it. The glandular condition of Meckel's diverticulum is particularly well-marked in the Woodcock {Scolopax rusticula). In that bird it is more than half an inch long, thick and very firm, with a narrow lumen slightly distended towards the apex and communicating with the cavity of the gut. The figure (text-fig. 44) shows part of a longitudinal section through the diverticulum. The epithelium lining the lumen, and continuous with that of the intestine, is thrown into a set of deep glandular folds forming a branching system that occupies nearly half the wall. In the interspaces between these tubular glands, numerous patches of lymphatic tissue occur. Towards the apex of the gland this secreting layer is much thinner and more regular, and there is less lymphatic tissue. The rest of the thickness of the wall of the gland is composed of longitudinal and circular muscle-fibres, rather irregularly arranged, and of loose connective-tissue stroma in which |