2l6 THE BEHAVIOR OF LOWER ORGANISMS. 



Swinging or bending movements take place with special frequency 

 while the pseudopodia are withdrawing ; in some Amoebae such move- 

 ments are an almost constant accompaniment of withdrawal. As it is 

 withdrawn the pseudopodium becomes roughened or warty on its sur- 

 face, as we have seen, and at the same time bends to one side or the 

 other, or swings back and forth. The impression given is that the 

 outer layer of the pseudopodium has become partially solid. In with- 

 drawing, the solid substance seems to melt gradually away, in a some- 

 what irregular manner, so as to leave solid masses connected by liquid 

 protoplasm, the projecting solid masses forming the wart-like roughen- 

 ings of the surface. When this melting away occurs more strongly on 

 one side, the pseudopodium bends at that point, toward the side which 

 has apparently become more fluid. 



We have in such a case, if appearances may be trusted, a mass com- 

 posed partly of solid, partly of fluid. While it is usually admitted that 

 parts of the protoplasm may become solid at times, little attempt has 

 been made to understand protoplasmic movements by studying the 

 physics of such mixtures of solids and fluids.* In certain experiments 

 with inorganic mixtures of this kind, in which movements were pro- 

 duced that resembled those just referred to in Amoeba, the writer 

 became convinced of the possible importance of the physics of such 

 mixtures for the understanding of protoplasmic activities. 



The experiments in question were concerned with the movements 

 under the action of surface tension of oil drops to which soot had been 

 added for the purpose of rendering the currents visible. When a large 

 quantity of soot was added, the drops became somewhat stiffened, and 

 now showed to a marked degree a mingling of the characteristic proper- 

 ties of fluids and solids. 



In one set of experiments clove oil was thus mixed with soot and 

 introduced as drops into a mixture of three parts glycerine and one part 

 95 per cent alcohol. The drops move about, as a result of local 

 decrease in surface tension, in the same manner as the olive-oil emul- 

 sion in Butschli's celebrated experiments. Much of the soot collects 

 next to the surface of the drop, and becomes massed in certain regions, 

 as a result of the currents, covering these regions with a sort of crust, 

 this crust being formed of separate solid particles. The particles are 

 crowded together as closely as possible, owing to the surface tension of 

 the fluid in which they are floating.f If the particles are not too minute 

 they may project above the surface of the drop, giving it a rough appear- 



* Some of the experiments of Rhumbler (1898, 1902) deal with such mixtures, 

 though not with a view to an understanding of the movements, but of certain 

 other processes. 



f According to the principles set forth by Rhumbler, 1898, p. 332. 



