FUNCTIONAL ClIANCJl-S IN STRUCTURIi 75 



could involve the transport of the iiitracistenial granules, or ol their material, to 

 other parts of the endoplasmic reticulum tree ot attached particles. Such a part 

 exists in the centrosphere region (the '(lolgi zone') of the cell where piles of 

 smooth surfaced cisternae and large numbers of smooth surfaced vesicles (fig. 6) 

 are regularly encountered (41, 30). At the periphery of the centrosphere region, 

 there are junctions between the rough- and smooth-surfaced parts of the system, 

 as indicated by the presence of elements of mixed appearance which could be used 

 to gain access from one part of the reticulum to another. Within the region itself, 

 many smooth-surfaced vacuoles or cisternae appear to be in the process of filling 

 (fig. 7) and all gradations are encountered between still flattened cisternae, partly 

 hlletl vacuoles, and fully packed, well rounded zymogen granules (8, 26). The 

 morphological evidence indicates clearly that the membrane of the zymogen 

 granules is derived from the centrosphere region, but is less informative about the 

 mechanisms whereby intracisternal granules gain access to the cisternae of the 

 (Jolgi zone. As already mentioned, there are connections between the rough-sur- 

 faced parts of the reticulum and the membrane-bound spaces of the centrosphere 

 region, but intracisternal granules are rarely seen in transit through such junc- 

 tions. For the moment, what appears to be of particular interest is the fact that, 

 upon discharge, the membrane of the zymogen granules becomes continuous with 

 the cell membrane at the apical pole of the exocrine cell. Through the exit thus 

 created, the content of the granule is poured out into the acinar lumen, which 

 frequently appears to be filled with discharged zymogen (fig. 8). By this simple 

 device the 'granules' can be discharged without any break in the continuity of the 

 cell membrane and without the risks involved by any membrane discontinuity. 



It appears, therefore, from these observations on the formation and discharge of 

 zymogen granules that membranous material picked up in the centrosphere re- 

 gion of the cell is continuously relocated in the cell membrane at the apical pole 

 of the cell. The implications of this finding are worth considering in some detail. 

 To begin with, it is highly improbable that the membrane moves only from the 

 centrosphere to the surface. An unidirectional movement would soon result in 

 considerable enlargement of the lumen and in exhaustion of the intracellular mem- 

 branous material. It is reasonable to assume that a concomitant movement, from 

 the surface to the centrosphere region, takes place, and the presence of small 

 'empty' vesicles below the luminal membrane is compatible with this assumption. 

 Observations made on other cell types, particularly on macrophages ( 19, 9, 31) 

 and amebae (14, 6, 4) clearly indicate that membrane material can be moved from 

 the cell surface to the interior. We may conclude that we 'see' the centrifugal arc 

 of this circular movement because the membrane is marked for its duration by 

 the characteristically dense content of the zymogen granules. Aiul we may assume 

 that we cannot visualize the centripetal arc because in its case we lack a natural 

 marker. In keeping with these general assumptions, we may further surmise that 



