PROTOZOA AS CELLS 37 



morphology of phagocytosis. Roth's (1960a) study confirms and 

 considerably extends the observations of several previous electron 

 microscopists (see section on amebae in Chapter 3). Food 

 vacuoles in protozoa soon after formation typically shrink by 

 elimination of ingested water, simultaneously becoming acid as 

 the prey is killed (Kitching, 1956b). Subsequent alkalinization 

 occurs, with some increase in volume, and ultimately the food is 

 reduced to an undigested residue surrounded by fluid; the vacuole 

 contents then are expelled through the cell surface. Correlating 

 his electron-microscope studies on Pelomjxa with light-microscope 

 observations by Mast, Roth identifies the following: (1) Recently 

 formed vacuoles containing unchanged food organisms and limited 

 water in a smooth, thin membrane from which the plasmalemma 

 fringe already has disappeared. (2) Early stages of loss of structure 

 of the food organism, when the vacuole membrane shows many 

 long villous protrusions and foldings toward the surface of the 

 prey. Roth believes that this appearance coincides with the 

 addition of alkaline substances to the vacuole from the ameba's 

 protoplasm. (3) Vacuoles found abundantly during the longer 

 phases of digestion and absorption of prey. A homogeneous 

 substance is layered against the vacuole side of the membrane, 

 and the latter, irregular in contour, is surrounded by a dense cloud 

 of tiny vesicles containing similar material (Fig. 14, PI. IV). 

 Budding of these vesicles from the membrane is suggested in 

 several micrographs, and Roth concludes that transfer of vacuole 

 contents into the cytoplasm by micropinocytosis is occurring. No 

 means of tracing the pinocytosis vesicles after their formation was 

 available. Roth emphasizes the enormous increase in membrane 

 area provided by such a process. If most of the contents of a food 

 vacuole are thus parceled out during a period of 12 hours, it 

 would be necessary that each /x 2 of original vacuole surface be 

 duplicated every minute. Interestingly, no special cytoplasmic 

 organelles such as mitochondria or Golgi membranes appear 

 regularly in the vicinity of the vacuole at any stage. 



A similar process of pinocytosis around phagocytosis vacuoles 

 is indicated in electron micrographs of the heliozoan, Actino- 

 sphaerium, studied by Anderson and Beams (1960). Jurand (1961), 

 in a study of food vacuoles in Paramecium, noted similar but less 

 abundant small vesicles surrounding food vacuoles presumably 



