PROTOZOA AS CELLS 33 



become constricted as strings of vesicles which may then decrease 

 in size. Instead of narrow channels, initial pinocytotic invagina- 

 tions may be wider, even as wide as food cups, so that the only 

 distinction between the two processes appears to be the in- 

 determinate one of food size — solutions or macromolecular 

 suspensions in the case of pinocytosis versus microscopically 

 visible particulate ingesta in phagocytosis. Intense pinocytotic 

 activity is stimulated in amebae by the presence of certain sub- 

 stances in the medium, notably proteins. The ultimate fate of 

 pinocytosis vesicles is unknown, and this is currently an important 

 mystery. 



Electron microscopy discloses apparently similar processes on 

 a more minute scale in many vertebrate cells as well as in protozoa. 

 Thus the engulfment of a bit of cell membrane surrounding a 

 droplet of ambient fluid occurs in a wide range of cells, and the 

 vesicles so formed vary widely in size. 



All of this has assumed particular interest in view of the known 

 absorption by living cells of molecules presumably too large to 

 diffuse across the cell membrane. Pinocytosis is an attractive 

 possibility as a partial explanation of this phenomenon, but the 

 fact remains that the contents of the pinocytosis vesicle are still 

 separated by a membrane from the cytoplasm. Does the membrane 

 rupture or decompose in the cytoplasm to release its contents? 

 Is the membrane permeability altered after engulfment? Does 

 digestion occur within pinocytosis vesicles, permitting subsequent 

 diffusion of the products ? Or does the increased membrane area 

 resulting from intense pinocytosis merely augment the total 

 quantity passed of a slowly diffusing substance ? In the case of 

 phagocytosis, digestion within a food vacuole has long been 

 believed to occur ; morphologic changes in the ingested food can 

 be observed microscopically, and changes in the pH of the vacuole 

 contents are clearly demonstrable. If similar processes take place 

 in pinocytosis vesicles, then it would seem necessary to conclude 

 that any bit of cell membrane is a potential digestive surface when 

 surrounded by any part of the cytoplasm. However, it may be 

 significant that the hairy plasmalemma coat of Amoeba seems to 

 persist for some time on pinocytosis vesicles or deep channels, 

 whereas it disappears very quickly (see below) from food vacuole 

 membranes. Opinions on these matters vary so much that at 



