352 DR. p. CHALMERS MITCHELL OX [DeC. 1, 



4. On the Occasional Transformation of MeckeFs Diverti- 

 culum in Birds into a Gland. By P. Chalmers 

 Mitchell, M.A., D Sc, Secretary to the Society. 



[Received December 1, 1903.] 

 (Text-figure 44.) 



Soon after I began to study the varying dispositions of the intes- 

 tines in Birds I, like other obsei-vers, was struck by the occasional 

 presence of a well-marked cfecum about the middle of the course 

 of the small intestine. This cfecum is identical morphologically 

 Avith 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 structui-e 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 ah-eady stated this fact 

 {loc. cit. p. 264) and mentioned that Lonnberg and Jagerskiold 

 have drawn attention to it ; but as the microscopic sti-uctui-e 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 pai-ticularly 

 Avell-marked in the Woodcock {Scolopax rusticida). In that bird 

 it is more than half an inch long, thick and veiy firm, with a 

 narrow lumen slightly distended towards the apex and communi- 

 cating 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 inter- 

 spaces 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 Avall of the gland is 

 composed of longitudinal and circular muscle-fibres, lather iri'Cgu- 

 larly arranged, and of loose connective- tissue stroma in which 



