^36 Comparative Animal Physiology 



expansion, cyclosis in Elodea, cleavage in Arhacia eggs-are similarly stopped 

 as the protoplasmic rigidity decreases under high pressures (Fig. 241). 

 These are in contrast to bioluminescence, ciliary movement, muscle contrac- 

 tion, and nerve propagation, which functions may be enhanced by pres- 

 sure properly applied.-^- ^^ 



The rate of locomotion is not directly related to the gel-sol ratio, even 

 though the gel is thicker posteriorly. For example, the gel increases with 

 increasing external acidity, it also increases as the temperature is lowered, 

 yet the rate of locomotion shows broad pH and temperature optima.^^^- ^^^ =^1 

 For brief exposures to different temperatures (not metabolic acclimation) 

 the velocity of locomotion in A. proteus rises from zero at about 5° C, to a 

 maximum at 22.5-24°, sometimes a double maximum, then declines to zero 

 at about 33°, '^^- ^'■* whereas in the marine Flahelhila the maximum is at 

 35-40°. The gel-sol ratio, however, declines with rising temperature,-^'-* as does 

 the viscosity (with some irregularity).^^ 



In Amoeba proteus there are two pH values at which the rate of locomo- 

 tion is maximal, pH 6.2 and pH 7.5, in the presence of a salt mixture; but 

 in the presence of single cations, Na+, K+, or Ca++, only one pH maxi- 

 mum occurs.^^ An amoeba which is largely sol, A. dnhia, showed an increase 

 in viscosity, as measured by centrifugation, in NaCl and KCl and a de- 

 crease in MgCL and CaCL., whereas, in similar concentrations on the gel- 

 rich A. proteus, Ca increased and K+, Na + , or Mg++ decreased viscos- 

 j{y_i3, 14 High external calcium tends to increase the gel-sol ratio in a pH 

 range where Na+ or K+ cause a decrease in gel.^^- "'•'^ The effects of salts 

 depend on external pH, but in no condition is gel-sol ratio or viscosity as 

 measured by centrifugation affected in the same manner as is the rate of 

 locomotion. The frequency of rupture of the plasmagel sheet is, however, 

 related to the gel-sol ratio; those conditions which decrease the amount of 

 gel also increase the frequency of rupture of the sheet.-^" Gel-sol ratio and 

 mean viscosity are important in locomotion, but other factors also contribute. 

 It is probable that the rates of gelation and solation are important; also the 

 contractility of the gel need not be proportional to its viscosity or to its 

 amount relative to the sol. Changes in elasticity of the gel as measured by 

 microdissection might be better correlated with rate of locomotion. How 

 external pH is effective on the gel even though the cytoplasm is well buffered 

 is not known. 



Locomotion can be altered by local solation or gelation. In an electric 

 field, for example, solation is induced in an amoeba on the side toward the 

 cathode and pseudopodia progress in that direction.'*- •^'' Intense illumina- 

 tion or mechanical stimulation of a pseudopod tip induces gelation and 

 streaming reverses in that pseudopod. -^^ Pantin^-- ^-^ suggested that con- 

 tractility of the posterior gel results from loss of water by syneresis and that 

 the anterior gel or freshly formed gel imbibes water. He claimed that 

 in a marine amoeba the granules at the anterior end and in active pseudo- 

 pods were more acid than those in the posterior end. This difference in 

 acidity was not confirmed by Mast.^^ However, immersion in hypertonic 

 solutions causes gelation and results in a rugose or wrinkled surface.'^*^* ^^ 

 Many amoeboid cells have at times an irregular tail-piece (A. Umax, P. pa- 



