GLANDULAR CELLS OF THE FROG'S PANCREAS 385 



It is necessary to say here a few words with regard to the 

 fixing and staining characteristics of prozymogen granules. In 

 mitochondria preparations, such as Altmann's, Benda's, and 

 Meves', they cannot be brought to light. The most distinct 

 pictures have only been obtained in preparations fixed in Miiller- 

 formalin and stained with iron-haematoxylin ; they can also be 

 exhibited, though not distinctly, by Zenker-formalin (without 

 acetic acid) fixation and iron-haematoxylin staining (fig. 77). 

 From these reactions it follows that prozymogen granules are 

 preserved, in a satisfactory manner, by the prolonged action 

 of potassium bichromate, and that they are destroyed by the 

 presence of acetic acid, even in a very small amount, in the 

 fixatives. This is a characteristic property of the granules in 

 question, by virtue of which they can be distinguished from 

 mitochondria and zymogen granules; and this might be also 

 the chief reason why investigators who employed fixatives 

 containing more or less acetic acid for exhibiting the mito- 

 chondria, did not notice the prozymogen granules within the 

 secretogenous area. 



The first-formed prozymogen granules are very small (fig. 73) ; 

 they gradually increase in volume (figs. 74, 75, 76, 78), probably 

 owing to their growth, but not to fusion with each other. 

 With increase in volume, they undergo changes in their stain- 

 ing reactions, thus becoming gradually transformed into typi- 

 cal zymogen granules. The young zymogen granules thus 

 formed can persist for some time within the secretogenous area 

 (figs. 75, 76, 78) ; they sooner or later leave it, however, to pass 

 into the surrounding plasma portion. Most of them pass 

 upward and lateralwards ; but there are some which proceed 

 toward the nucleus (fig. 76). This indicates that there is no 

 uniformity as to the size of zymogen granules with respect to 

 their position ; the occurrence of the larger ones near the nucleus 

 thus being explained. If the passing out of the granules of the 

 area is, for some reason or other, retarded, they will continue 

 to increase in volume, so as often to lead to the formation of 

 larger zymogen granules within the area (figs. 75, 78). From 

 the above description, it is evident that there is a marked dif- 



