550 



ROBERT D. ALLEN 



Third, a new concept of amoeba cytoplasmic structure has emerged 

 from recent rheological studies of consistency differences in various parts 

 of the moving cell [2, 3, 7]. In contrast to the traditional "sol-gel" concept 

 of amoeba structure, it has been shown that the axial portion of the 

 endoplasm (Mast's "plasmasol" [14]) possesses weak gel structure. 

 Velocity profiles of endoplasmic streaming within the ectoplasmic tubes 

 of narrow, cylindrical pseudopodia of Chaos chaos were found to be 

 similar to those found for plug flow of a non-Newtonian fluid in a tube [7], 

 and were also very similar to velocity profiles of cytoplasmic streaming in 



----HYAtlNE CAP 



- FOUNTAIN ZONE 



---- PLASMALEMMA 



AXIAL ENDOPLASM 

 -SHEAR ZONE 

 ■ HYALINE ECTOPLASM 



GRANULAR 

 FCTOPLASM 



RECRUITMENT 

 ZONE 



TAIL OR UROID 



Fig. I. A diagrammatic representation of the concept ot amoeha pseudopodial 

 structure discussed in the text. The superimposed curves are velocity profiles from 

 the data of Allen and Roslansky [7]. 



myxomycete channels [13]. Plug flow occurs when the shear stress acting 

 on a fluid near the centre of a stream is insufficient to cause significant 

 rates of deformation (i.e. velocity gradients) in the fluid, but when the 

 higher shear stresses near the walls exceed the yield point of the material 

 (if it is a gel) its apparent viscosity is reduced to the range expected of true 

 sols. The velocity profiles, by demonstrating the quasi-pseudoplastic 

 nature of endoplasmic flow, have drawn attention to the presence of weak 

 gel structure which might permit the development and transmission of 

 tension. Only the tail endoplasm and the peripheral endoplasm of the 

 "shear zone" (Fig. i) ha\'e shown evidence of a low apparent viscosity. 

 Studies with the centrifuge microscope have recently confirmed the 



