A new description is given of certain structures in Petromyzon and Myxine which, in view of their anatomical relationships and developmental history, have been considered by some earlier workers to be comparable with the pancreas of higher vertebrates.

They are composed of cords of cells which develop, at least in part, from the bile duct. In Petromyzon some of them are shown to be closely associated with a small caecum of the intestine.

They are not externally secreting structures, and in particular they do not in either animal communicate with the intestinal lumen, but there is histological evidence, more convincing in Petromyzon than in Myxine, that they may be endocrine organs.

In both animals secretory cells are present in the intestinal epithelium, and may be considered to represent the zymogen cells of the pancreas. Interpretation of the cords as islet tissue, however, cannot be established on histological grounds alone. Using the Helly-Azan technique, which successfully differentiates the cellular components of the latter in fish, it is shown that only one type of cell is present in the cords; this does not give the characteristic staining reaction of A cells, and differs in certain respects from the B cells.

The significance of these results is briefly discussed in the light of the evolutionary status of the Cyclostomata.

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