PROTOZOA AS CELLS 11 



low-molecular- weight solutes were withdrawn during dehydration 

 of the specimen. In a significant and meticulous recent study on 

 amebae, Wohlfarth-Bottermann (1961) has applied a series of 

 different fixing agents to cells grown under identical conditions. 

 These included the conventional osmium tetroxide, osmium 

 tetroxide plus potassium dichromate, potassium permanganate, 

 formalin alone, and formalin preceded or followed by osmium 

 tetroxide. He found that the same recognizable structure, and 

 variation in structure, can be demonstrated following any one of 

 these fixatives. The appearance of the hyaline cytoplasm varies, 

 within any single cell, from nearly homogeneous through a 

 condition marked by the random dispersion of minute (8 to 9 m^u) 

 globular particles to a compact net-like arrangement resulting 

 apparently from the linear aggregation of globular particles, 

 which now appear larger and denser. The last-named aspect he 

 believes represents a state of gelation or higher viscosity of the 

 cytoplasm. 



One class of cytoplasmic granules is of particular significance. 

 These are dense, irregularly angular in shape, and 10 to 15 m/x in 

 diameter (Fig. 1, PI. I); often they are disposed on membrane 

 surfaces. The microsome fraction of centrifuged cell homogenates, 

 which has distinctive biochemical properties, is composed largely 

 of particle-studded membrane fragments and free particles. 

 Palade and Siekevitz (1956a, b) first succeeded in identifying these 

 particles, from rat liver cells and from pancreatic exocrine cells 

 of the guinea pig, as ribonucleoprotein (RNP), and it is now 

 generally agreed that they represent the principal basophilic com- 

 ponent of cytoplasm. It cannot be assumed, of course, that all 

 granules of this description are RNP, and not all ribonucleic acid 

 (RNA) is confined to the particles, but correspondence between 

 particle frequency and cytoplasmic basophilia is a general rule. 

 Whether the particles represent a precipitate or are present as 

 such in life is not certain. Dense, angular particles within the size 

 range 1 to 1 5 m/x or thereabout will be referred to here descriptively 

 as Palade particles, without intending to imply that their 

 composition is known. 



Scattered through this matrix are membrane-enclosed compart- 

 ments ranging from tiny vesicles and canaliculi through extensive 

 flattened sacs to large vacuoles, in a continuous spectrum of sizes 



