140 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



orientation and rarely in contact with the limiting membrane. 



In a site anterior to the kinetoplast and lateral to the reservoir, 

 a Golgi element usually is seen. Since it does not appear to be 

 attached to the kinetosome, it is a dictyosome rather than a true 

 parabasal body. A larger vesicle in this region may represent the 

 contractile vacuole, but it lacks distinctive cortical structure. The 

 cytoplasmic matrix is fibrogranular and contains assorted profiles 

 of endoplasmic reticulum. The nucleus has a perforate double 

 envelope and a dense central nucleolus. 



Two novel structures of particular interest have been reported : 

 a basophilic, rod-shaped body in the cytoplasm of Crithidia 

 (StrigoMonas) oncopelti (Newton and Home, 1957) and a fibrous 

 tube tentatively identified as a cytostome in Trypanosoma mega 

 (Steinert and NovikofT, 1960). Newton and Home were able to 

 isolate the rods, which always were located singly or in pairs in 

 the posterior end of the cell. Analysis showed that they consisted 

 largely of ribonucleoprotein, containing about 10 to 15 per cent 

 of the total cell RNA. Apparently they reproduce by fission 

 in situ. No evidence was available concerning their significance. 



The "cytostome" demonstrated by Steinert and NovikofT in 

 cultured crithidias of T. mega consists of a cylinder of fibrils 

 apparently continuous with the pellicular fibril system, opening 

 near the reservoir and curving deep into the cytoplasm; its internal 

 terminus has not been identified. The cell membrane is depressed 

 at the outer opening of the tube, forming a conical pit. Most of 

 the length of the tube is thus filled with cytoplasm, but occasionally 

 within it and frequently in linear arrays near it are found vesicles 

 of about 180 niju, diameter, outlined by distinct unit membranes. 

 By addition of ferritin to the culture medium, Steinert and 

 NovikofT were able to show a concentration of ferritin molecules 

 over the cell membrane at the open mouth of the tube, and also 

 within the vesicles just described. The authors concluded that 

 ingestion of ferritin by pinocytosis through the "cytostome" was 

 occurring ; confluence of ferritin-carrying vesicles formed larger 

 inclusion bodies in the posterior end of the cell. 



Although phagotrophy by trypanosomes has been suggested 

 in the past, most investigators have remained skeptical; the 

 organisms were assumed to survive by absorption of nutrients 

 from blood and tissue cells. The common occurrence of micro- 



