32 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



create enough hydrostatic pressure to displace the surrounding 

 cytoplasmic mass, yet fluid continues to enter. 



Contractile vacuoles in flagellates have been seen occasionally 

 in electron micrographs; in several instances, simple membranes 

 with no apparent cortical differentiations have been described. 

 This is true of several phytoflagellates where contractile vacuoles 

 are conspicuous in life, and a highly differentiated cortex, if 

 present, would surely have been observed by the investigators. 

 Gatenby, Dalton, and Felix (1955) have pictured a lamellar body 

 resembling the conventional Golgi apparatus bordering the con- 

 tractile vacuole of some flagellates, and they cite this in support 

 of Nassanov's hypothesis (disputed by Grasse and Hollande, 1941, 

 among others) of the homology of protozoan contractile vacuoles 

 with the Golgi complex. 



The clearest micrographs of flagellate contractile vacuoles are 

 those of Paraphysomonas vestita by Manton and Leedale (1 961c). The 

 bounding membrane is somewhat convoluted in the pictures 

 shown, and around it are many smaller vacuoles, but no layer of 

 packed convoluted tubules. The Golgi body lies nearby, but its 

 closest edge appears to be nearly a micron away from the vacuole. 

 Golgi microvesicles are similar to the small vesicles surrounding 

 the contractile vacuole, so a migration from the Golgi zone is not 

 inconceivable, but neither is it strongly supported by the available 

 evidence. In many flagellates Golgi and contractile vacuole both 

 occupy fixed positions near the kinetosomes, but evidence to 

 suggest a direct functional relationship is lacking. 



Pinocytosis and Phagocytosis 



The phenomena of phagocytosis and pinocytosis are treated 

 together because current evidence suggests that similar processes 

 are involved, even in species that have permanent mouth 

 openings. 



Pinocytosis or " cell drinking " was first described (for historical 

 citations and review, see Holter, 1959) by Lewis in 1931 in cells in 

 tissue culture. Mast and Doyle in 1934 identified a similar process 

 in amebae, and in recent years the phenomenon has attracted 

 wide interest. At the light-microscopic level, pinocytosis in 

 amebae typically involves the formation of many slender channels 

 from the cell surface deep into the cytoplasm. The channels 



