STUDIES ON THE PANCREAS OF THE GUINEA PIG 355 



of islet cells by negative characters. We are now able to sub- 

 stitute for this a positive definition and to consider in the light of 

 such a definition the transitions which have been described. I 

 shall describe first the zymogenic cell, then contrast with this 

 the two types of islet cells, and those of the smaller ducts, and 

 centroacinous cells. 



1. The cells of the pancreatic acini 



In the structure of these cells the following elements require 

 consideration: zymogen granules; prozymogen granules; the baso- 

 phile substance; mitochondria; internal reticular apparatus; the 

 continuous substance of the protoplasm; and the nucleus. 



When a preparation of the fresh pancreas of the guinea pig 

 mounted in guinea pig serum is examined under a high power 

 the characteristic division of the pancreatic cell into two zones is 

 seen. The central zone of the cell, varying in width with the 

 stage of functional activity, is closely packed with coarse granules 

 which are usually known as zymogen granules. The basal zone, 

 under the low power, or under high power dry lenses, appears 

 perfectly transparent and homogeneous, though small fat drop- 

 lets may be present. Under a good apochromatic oil immersion 

 objective, this basal zone exhibits an indistinct striation, paral- 

 lel in direction to the long axis of the cell and perpendicular to 

 the basement membrane. This striation was first observed by 

 R. Heidenhain, and is due, as will appear later, to the presence of 

 long mitochondrial filaments or rods (chondrioconts) similar to 

 those demonstrated in the tissues of embryos by Meves ('08). 

 The substance between these filaments appears homogeneous in 

 the fresh cell even under the best apochromatic objectives, and 

 exclusive of the mitochondrial filaments mentioned above exhibits 

 no structural elements whatever. It should be pointed out here 

 that the mitochondrial filaments have no relation to the so-called 

 basal filaments of Solger which belong to the homogeneous basal 

 substance. Under certain conditions, there may be seen in the 

 fresh cell, better in cells stained with neutral red, at the junction 

 of the basal zone and the aone of zymogen granules an intermediate 



