354 R. II. BENSLEY 



of islets with ducts in the guinea pig may dispel these doubts by 

 an experiment which takes less than twenty minutes to perform. 

 The source of the idea that islets of Langerhans are separate 

 from acinus tissue, and surrounded by a special capsule is to be 

 found in the islets of class 2, which have both of these characters 

 because, though contained within the lobule, they are not really 

 part of it, but have intruded into it from outside. These islets 

 are among the most conspicuous of intralobular islets, and be- 

 cause of their sharp separation from the surrounding acini, and 

 also of their size, are most likely to be regarded as ' typical' islets 

 and so selected for study by those investigators, who have not at 

 their disposal a technique which permits of the accurate diagnosis 

 of islet cells. It is true nevertheless that the majority of these 

 islets are connected with ducts, though the same obstacles to the 

 discovery of these connections are present as in the case of the 

 interstitial islets. 



THE STRUCTURE OF THE PANCREATIC EPITHELIUM 



Having considered from the standpoint of quantitative study 

 the claims that have been advanced with respect to the possi- 

 bility of transforming acinus tissue into islet tissue by experiment, 

 with results which are wholly opposed to these claims, it remains 

 to consider the other evidence of transformation which has been 

 brought forward, namely the statement that, between the cells 

 of the acini on the one hand and those of the islet on the other, 

 are to be found cells of intermediate characters, which may be 

 regarded as transitional elements. If such transitions occur, 

 then the quantitative results merely indicate that a balance is 

 maintained between the construction and deconstruction of islets. 

 It is necessary, therefore, to consider in detail the descriptions of 

 transitions which have been given, and to inquire to what extent 

 each satisfies the logical requirements of the case. Before pro- 

 ceeding to this discussion, however, it is necessary to know the 

 nature of the cells between which we are seeking transitions. It 

 has been pointed out in the introduction to this paper that, with 

 few exceptions, workers in this field have accepted a definition 



