Fig. 2. The micrograph shows at a high magnification a small field in the basal region 

 of a pancreatic exocrine cell (rat). 



Parts of two mitochondrial profiles appear at m; the rest of the field is taken by numerous 

 profiles of the endoplasmic reticulum (rs), most of which are of elongated form and appear 

 disposed in parallel rows. In three dimensions many of these profiles correspond to rela- 

 tively large but flat vesicles known as cisternae. 



The membrane limiting the cavities of the reticulum appears as a dense, fine line (n) 

 whenever sectioned normally, and as a less dense, poorly outlined band (o) when cut 

 obliquely. In a few places the section has opened small windows (/) in the wall of the 

 cisternae. 



The membrane of the endoplasmic reticulum separates two distinct phases in the 

 cytoplasm: one is represented by the light, homogeneous material enclosed in the cavities 

 of the system (c); the other, by the surrounding cytoplasmic matrix (mx). 



Numerous small, dense particles, — 150 A in diameter, appear attached to the outer 

 surface of the membrane limiting the cavities of the endoplasmic reticulum (ap). In ad- 

 dition to these attached particles, particles of comparable size and density occur apparently 

 freely scattered in the cytoplasmic matrix (fp). Note, however, that many of these particles 

 form short chains (arrows) anchored with one end among the attached particles. 



Fixation: 24 hours at room temperature in 1 per cent Os0 4 in acetate veronal buffer, 

 pH 7.6. As a result of the long fixation, part of the cytoplasmic matrix has been extracted 

 and the profiles of the endoplasmic reticulum and the small, dense particles appear in 

 better contrast. 



Embedding: n butyl methacrylate. 



Magnification: 50,000. 



