On the Relations between the „Islets of Langerhans" etc. 75 



lightly than the zymogenoiis tissue. Each cell is elongated-columnar, 

 or ciescentic in form and extends completely across from one side of 

 the column or mass to the other. Several cells form a kind of whorl, 

 their bases being narrow and in close contact, while their apices extend 

 over a considerable area. The nucleus is large and distinctly oval, 

 the protoplasm is finely granular and vacuolated. The whole group 

 of cells is surrounded by a wide capillary network. Other islets are 

 composed partly of elongated cells, and partly of polyhedral cells 

 (PI. V, fig. 14, i). These islets can be seen in nearly every case to be 

 continuous in some directions with the zymogenous tubules (PI. V, fig. 14, 

 trans, c), and are often arranged in a radial fashion round a capillary 

 vessel. There are , also many islets of the branching columnar form 

 bearing a general resemblance to those described in the dog (see pp. 3 — 6). 



In the amphibians we have noticed a very significant tendency 

 for certain columnar cells to be arranged in a palisade fashion round 

 the veins great and small (PL V, fig. 15, PI IV, fig. 16). This we 

 suggest may be an anatomical expression of an internal secretion. 



In the newt (Plethodon glatinosus) this arran2'enient is com- 

 moner than in the frog (see PI V, fig. 15, PI. IV, fig. 16) and at 

 first we were inclined to think that it constituted the only homologue 

 of the leptochrome islets in other animals. But, in th^i frog there are 

 to be seen leptochrome islets often consisting of a single solid column 

 of faintly staining cells, continuous with the zymogenous alveoli and 

 only distinguishable from them by the lighter tint of the cells, the 

 specially wide capillaries on either side, and the absence of a lumen 

 (This is a reduction of what is shown in PI. V, fig. 14.). In the 

 newt the islets are still further reduced, so that they consist in most 

 cases of a circle of lai-ge, pale, more or less columnar cells surrounding 

 a capillary blood-vessel (see PI. IV, fig. 16, i.). 



The bathychrome tissue in the specimens of Plethodon we have 

 examined seems to be confined to a portion of the pancreas which is 

 embedded in the wall of the intestine. This portion appears in many of 

 our sections to consist almost entirely of the darkly staining tissue^). 



*) Since the above was witten I have observed examples of bathychrome tissue 

 in other portions of the pancreas of Plethodon. 



