Amoeboid Movement 637 



lustris, mammalian lymphocytes), from the gel of which it is likely that water 

 has been removed. If the gel matrix consists of long polypeptides cross-linked 

 by side-chains, shortening or contraction would involve the loss of water.^- ^ 

 Another suggested mechanism for solation is that it results from the thixo- 

 tropic character of the gel, such that the mechanical agitation caused by 

 contraction leads to solation, while gel reforms as soon as movement ceases 

 at the anterior end.*^ 



Elastic tension might be developed if there were a decrease in volume of 

 the gel as it sets, as there is in gelatin. However, gels which solate under 

 pressure as does the gel of amoeba, increase slightly in volume on setting,-** 

 hence the contractile tension can hardly result from a volume decrease. It 

 is more likely that fibrous proteins or polypeptide chains shorten according 

 to their state of hydration. 



The contractile elements in the plasmagel may resemble contractile muscle 

 proteins. The "viscosity" of actomyosin decreases with pressure, as does the 

 amoeba gel viscosity. In amoebae two processes must be separated: (1) the 

 continuous solation at the posterior end and gelation at the anterior end, 

 and (2) the persistent contractile tension of the plasmagel. In muscle there 

 are two different processes: (1) the activation of the contractile elements, 

 probably via adenosinetriphosphate (see Ch. 16, p. 605), and (2) the 

 rapid development of tension in the gel. The elements of the gel matrix 

 of amoeboid cells must be much less regularly organized than those in 

 muscles because an amoeba shows no birefringence except for a few gran- 

 ules. However, the axopods of Hehozoa and Radiolaria are very contractile 

 and are birefringent.^^ These axopods may well represent an intermediate 

 state of organization between the weakly contractile gel of rhizopods or 

 myxomycetes and true muscle. 



The difference in the effect of pressure on muscle and amoebae may be 

 explained as follows: solation in amoebae may result from either blocking 

 the gelation process or reducing gel rigidity or organization; in muscle the 

 enhancement of contraction may be due to the effect of pressure on the 

 process of activation of the contractile proteins. This interpretation is sup- 

 ported by the fact that if pressure is applied after. the beginning of con- 

 traction in muscle the tension is actually reduced.^ It would be of interest 

 to know the effect of pressure on contraction of actomyosin fibers and on 

 the axopods of Heliozoa and Radiolaria. 



The ejiergy basis for the gel contractility in amoeboid movement is un- 

 known. Amoebae can move for long periods in the absence of oxygen, but 

 the rate of locomotion gradually declines.^^- -- Newly formed pseudopods 

 are more sensitive to cyanide than are the posterior regions.^'' Enucleated 

 fragments of amoebae round up and cease movement. 



The preceding evidence indicates that, although there may be minor 

 variations in amoeboid movement with different cell and pseudopod types, 

 the basic pattern is the same in all amoeboid cells. Velocity of locomotion 

 appears to be more closely related to the elastic tension of the plasmagel than 

 to the amount or viscosity of the gel. The gel may contain polypeptide 

 chains which are in a contracted state but which are less organized than 

 corresponding chains in muscle. Detailed information is lacking regarding 



