STUDIES ON THE PANCREAS OF THE GUINEA PIG 369 



cell but are usually more elongated. In the A cells also the gran- 

 ular content is much more variable than in the B cells, and one 

 may find A cells which differ from the clear cells just described 

 only in the fact that they contain a few fuchsinophile granule, 

 and all gradations of granule content up to cells which are crowded 

 with granules. This suggests a possible source for the A cells, 

 namely that they may arise by differentiation of the clear cells 

 which are included in the islet from the time of its formation. 



In point of frequency the B cell far exceeds the other two, 

 •forming the major part of the bulk of all the islets. Many of the 

 smallest islets consist of B cells with a few of the clear cells de- 

 scribed above (figs. 5 and 9). This is true of the small islets orig- 

 inating from the duct which contain no A cells, though the larger 

 islets of this group contain large numbers of them. The A cells 

 and clear cells are not confined to any particular portion of the 

 islet as supposed by some, but occur in the interior as well as on 

 the surface (fig. 13). 



Whether the two main types of islet cells are developmental 

 stages of a single element or not, it is difficult to determine. 

 The suggestion of Tshassonikow ('06) that the A cells were inter- 

 mediate in character between the cells of the acinus and the B 

 cells, I believe to be untenable for the following reasons: first, 

 I have shown that they occur in large numbers in the large inter- 

 stitial islets which have developed from the ducts without passing 

 through an acinus phase; second, they are located in large numbers 

 in the interior of the islet as well as on the surface where one might 

 expect to find transitional elements; third, they are present in 

 large numbers in the acini of new born guinea pigs unassociated 

 with B cells, whence they disappear within the first week without 

 a proportionate increase of B cells; finally the assumption that 

 they are intermediate between acinus cells and B cells is based on 

 another assumption that they present intermediate characters 

 between these two types which is not true. As a matter of fact, 

 A cells are as far different from acinus cells as are B cells, as Lane 

 has clearly shown. 



A second possibility that they are more advanced stages in the. 

 history of the islet cell than the B cell has more facts to supp( rt 

 it, for I have shown that the small islets associated with the duct 



