68 Swale Vincent and F. D. Thompson. 



statement that the islets are more abundant in the rabbit than in the 

 dog or cat. We find fewer in the rabbit than in the carnivora. 



The islets consist of a network of branching solid columns of 

 cells, apparently for the most part only one cell deep and presumably, 

 therefore, not more than half the width of the zymogenous acini. 

 When stained with haematoxylin and eosin the islet comes out a pur- 

 plish pink as opposed to the dark purplish blue of the rest of the 

 tissue. The protoplasm of the islet cells seems almost homogeneous. 

 The columns of cells are seen to be continuous in certain places with 

 the secreting alveoli. 



The diameter of the islet columns averages 12 ^<, that of the 

 alveoli 23;«; the diameter of the nuclei in the islets is about 4 /<, of 

 those iu the tubules 6 — 8 .u. The capillaries are very large. 



The islets of Langerhans in the guinea-pig. 



It is well known that the islets in the guinea-pig are so large 

 as to be visible to the naked eye. On microscopical examination with 

 the low power they stand out as large, rounded, oval, or irregular 

 patches of lightly staining material. Tiansitions to zymogenous tissue 

 are so common that in the case of the majority of islets, it is quite 

 impossible to define their limits under a high power, since the two 

 tissues fade gradually into each other. There is a comparatively wide 

 zone of cells which partake in varying degrees of the characters of 

 zymogenous and islet cells. On the other hand, in the central portion 

 of the islet, what we may regard as the most typical islet structure 

 is shown more beautifully than in any other mammal we have examined. 

 It consists of branching and anastomosing columns two or more cells 

 deep continuous at the periphery of the islet with the secreting alveoli. 

 On a first glance at a section the islet is veiy clearly indicated, 

 especially in an eosin preparation, by the usual wide capillaiy network. 



Scattered throughout the preparation dark cells are seen eithei' 

 singly or in small groups in the zymogenous acini. These are the 

 bathychroiiKt cells which have lieen alr(iady desciibed in olher animals. 



We have, also examined tlic paiicieas of tln^ chipmunk, the mouse, 

 and the liai. In I lie cliipmnnk islets are (•(impanitively inric(|nenl. in 



