Fig. 1. The electron micrograph shows part of two adjacent exocrine cells in the 

 pancreas of a guinea pig. The apposed membranes of the two cells appear obliquely sec- 

 tioned at cm. 



The basal region of one of these cells occupies the lower half of the figure and char- 

 acteristically contains a few mitochondrial profiles (m) and numerous profiles of the 

 endoplasmic reticulum (rs), which belong to the rough-surfaced type and show a certain 

 amount of preferred orientation, i.e., are disposed in more or less parallel rows. 



The apical region of the cell, in the upper part of the figure, is occupied by a few circular 

 profiles of zymogen granules (z) . Their dense content consists of stored digestive enzymes 

 and enzyme precursors. Elements of the endoplasmic reticulum appear disposed at random 

 among the zymogen granules. Part of the centrosphere region of the cell, with its char- 

 acteristic clusters of smooth-surfaced vesicles, can be seen at cs. 



A region similar to the lower right quarter of this figure appears in figure 2 at a higher 

 magnification. 



Specimen fixed for 2 hours at 0° C in 1 per cent Os0 4 in veronal acetate buffer, pU 7.6, 

 containing ~5 per cent sucrose; embedded in n butyl methacrylate. 



Magnification: 24,000. 



