88 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



(Mercer and Shaffer, 1960). Small spherical vacuoles are identified 

 by the authors as contractile vacuoles, but these lack a structurally 

 specialized cortex and are not associated with mitochondria. 

 Mitochondria with typical twisted microtubules and a rather dense 

 matrix are present, and occasional clusters of tiny vesicles suggest 

 a resemblance to a Golgi apparatus. The nucleus is rounded, with 

 a conventional double membrane and several dense nucleoli 

 occupying dome-shaped bulges of the nuclear surface. Vacuoles, 

 often with ruptured walls, that enclose bacteria in various stages 

 of digestion are abundant in feeding and migrating amebae but 

 disappear following aggregation. In these digestive vacuoles the 

 bacterial cells become swollen and the cytoplasm disintegrates, 

 but the cell walls remain and are surrounded by whorls of con- 

 centric dense membranes which ultimately appear to be ejected 

 from the cell. Small, dense, homogeneous crystalline bodies appear 

 in cells following aggregation. 



The cytoplasm of the solitary cells is bounded by a distinct unit 

 membrane. In the aggregate, cells are packed tightly together, 

 with their slightly irregular surfaces separated by a rather constant, 

 narrow (about 20 m/x) gap. Occasionally, small areas of higher 

 density underlying the apposed membranes are seen, and Mercer 

 and Shaffer pointed out a similarity with developing desmosomes 

 of metazoan epithelial cells. However, no additional material is 

 observed between the membranes, and permanent desmosomes 

 would hardly be expected so long as the cells are free to shift 

 positions within the aggregate. 



Gezelius and Ranby (1957) and Miihlethaler (1956) investigated 

 the structure of the fruiting body of Dictyostelium and found a 

 cellulose stalk membrane with fiber structure similar to the 

 cellulose of higher plants but of a lower crystalline order. 



Electron micrographs of the plasmodia oiVhysarum polycephahim 

 by Harris and Mazia (1959) and Stewart and Stewart (1959, 1961) 

 agree in showing a rather dense fibrogranular cytoplasm without 

 evident reticular membranes. Many clear vesicles of all sizes and 

 shapes appear. Stewart and Stewart (1961) suggest that these 

 vesicles may contribute membranes to the cell surface; an isolated 

 droplet of extruded cytoplasm is seen in electron micrographs to 

 have a highly convoluted surface membrane, underlain by a zone 

 of homogeneous cytoplasm nearly empty of vesicles. Furthermore, 



