364 R. R. BENSLEY 



as a network of closed spaces apparently excavated in this baso- 

 phile material. The apparatus is located in the portion of the 

 cell between the nucleus and the lumen but branches of the canals 

 may extend basalwards along the sides of the nucleus. I have 

 not been able to find any trace of a communication of these canals 

 with pericellular spaces, either in these preparations or in sections 

 prepared by the methods recommended by Holmgren and Kopsch 

 and Golgi for the positive demonstration of the canals. Nor 

 have I seen any continuation into them of processes of cells of the 

 basement membrane or of centroacinous cells. I am therefore 

 of the opinion that they constitute a system of closed spaces. I 

 have elsewhere ('10) brought together observations in support of 

 the theory that the canals of this system are the homologue in the 

 animal cell of the vacuole which is so important an organ of the 

 plant cell. 



Apart from the filaments of Altmann (mitochondria) and the 

 basal filaments I have not been able to demonstrate any structural 

 elements of a fibrous nature, or any alveolar structure in the pro- 

 toplasm of the acinus cell. 



The nucleus of the acinus cell is spherical, and contains in addi- 

 tion to several large chromatin masses a well defined large oxy- 

 phile nucleolus. 



2. The structure of the islet-cells 



The islets of Langerhans of the guinea pig as shown by Diamare 

 ('99),Schulze ('00),Dewitt ('06), and Lane ('07), consist of at least 

 two types of cells, which Lane calls the A cells and B cells respec- 

 tively. Many of the small islets are composed of B cells, which 

 also are the most abundant cells in all the islets. 



In the guinea pig, on account of the great size of some of the 

 islets and on account of the fact that many of them occur in the 

 interstitial tissue, it is easy to pick out an islet, detach it from 

 the surrounding tissue and examine its cells in isotonic salt solu- 

 tion or in native serum. Under these circumstances the cells of 

 the islet are seen to be crowded with extremely fine granules, 

 which have a low refractive index, and so are difficult to see except 



