370 



R. R. BENSLEY 



system contain at first no A cells, while the large islets contain 

 them in large numbers. They must either have been formed from 

 the B cells or from the undifferentiated cells which also form a 

 part of the small islets. Unquestioned transitions between the 



.:^1 



^*~a. 



Fig. 13 An interstitial islet of the guinea pig showing distribution of the A 

 cells, B cells and undifferentiated cells in the middle section of a series. Acetic 

 osmic bichromate preparation stained in anilin acid fuchsin and methyl green; 

 a, A cells; d, undifferentiated cells. The general tint indicates the mass of B cells. 

 X 334. 



B cells and the A cells I have not been able to find in my prepar- 

 ations, though, on account of the complex arrangement of the 

 cell columns in the islet it is not difficult to find cases where a 

 A cell is covered by a thin section of a S cell and so appears at 



