76 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



granules, streaming may readily be watched in the light micro- 

 scope, and it always occurs in two directions simultaneously in 

 every pseudopod. Granules pass outward along one side of a 

 pseudopodium and, if it has a free end, turn around and pass 

 inward along the other side. Multiple paths of streaming are 

 visible on thicker basal pseudopodia and on protoplasmic nodes 

 within the reticulum. Constant bending, twisting, lateral move- 

 ment, splitting and joining of pseudopodia occur, but the two-way 

 streaming is rarely if ever interrupted. Jahn and Rinaldi suggest 

 that all but the finest pseudopodia are fascicles of finer units and 

 that a fundamental streaming unit of probably less than 1 fi 

 diameter exists. 



The pseudopodia of Allogromia possess considerable rigidity, 

 since they extend unsupported through the medium and do not 

 collapse under the impact of collision with an errant ciliate. 

 Granules in a single streaming path move all at the same velocity, 

 like steps in an escalator. There is no evidence of any axial 

 supporting structure nor of an internal, more fluid zone. Jahn 

 and Rinaldi conclude that active shearing forces operate between 

 paired hemicylindrical filaments in each pseudopod. The 

 mechanism for such a continuous shifting of two gel surfaces 

 against each other is unknown, but possibly would have some- 

 thing in common with the phenomenon of sliding filaments 

 believed to be responsible for the contraction of striated muscle. 

 Similar processes may operate wherever cyclosis occurs in plant 

 and animal cells, provided that the streaming mass has a relatively 

 high viscosity so that a shearing force could act between its 

 surface and a fixed cortical gel. 



Both Allen's and Jahn's explanations of rhizopod movement 

 require the assumption that streaming units possess considerable 

 structural integrity. The stage would seem to be set for some 

 critical investigations combining electron-microscope studies of 

 ultrastructure with other biophysical techniques — if only the 

 electron microscopists can discover some meaningful structure! 



For the most part, results to date have been disappointing, as 

 author after author has reported no detectable difference between 

 solated and gelated parts of the cytoplasm of Amoeba and Pelomyxa. 

 The most enlightening reports yet available are those by 

 Wohlfarth-Bottermann (1960b, 1961) cited in Chapter 2, of 



