THE MOVEMENTS AND REACTIONS OF AMCEBA. 217 



ance, similar to that of the withdrawing pseudopodium of Amoeba. 

 Such parts of drops or whole drops, so covered, show peculiar proper- 

 ties. The form may be changed by lowering the surface tension locally 

 or by mechanical action from outside, exactly as in a fluid, but there 

 is an inclination to hold a form once received. The tendency to take 

 the spherical form is still somewhat marked, and if the irregular drop 

 is strongly disturbed it frequently slowly becomes spherical. On the 

 other hand, if not strongly disturbed, it may retain almost any form 

 impressed upon it cylindrical, flattened, irregular, or with long, slender 

 projections. In this power of receiving and retaining an irregular form, 

 yet with a tendency to become spherical, these drops, of course, resem- 

 ble Amoaba. These properties vary with the amount of soot present in 

 the oil ; if this is less, the drops slowly return to the spherical shape 

 when deformed ; if greater, they retain the irregular shape indefinitely. 

 Such irregular masses nevertheless flow together if brought in contact, 

 will quickly gather together into a close mass if strongly deformed, 

 and in many other ways they show the characteristics of fluids. 



The reason for their tendency to retain irregular forms is obvious. 

 The surface is covered with small solid particles that are in contact. 

 The projection of these particles above the surface may cause a rough- 

 ening of the surface. Any change of form, such as surface tension 

 would produce, causes much friction between these particles. The 

 form taken is, then, a resultant of the action of surface tension and the 

 resistance of these particles to movements.* Similar forms are pro- 

 ducible by mixing soot with bone oil and studying drops of the mixture 

 in a vessel of glycerine. 



For our purpose the phenomena which occur when the soot particles 

 are unequally distributed are of special interest. Consider an elon- 

 gated projection, as in Fig. 78, A, with the surface entirely covered 

 with closely crowded soot particles except in a certain region, x-y, on 

 one side. In this region x-y surface tension will have free play, tend- 

 ing to draw the points x and y together, while elsewhere the tendency 

 to contraction will be resisted by the friction of the particles. The 

 result is that the points x-y are drawn together, and the projection 

 bends toward that side. Since this bending does not tend to crowd 

 the particles on other parts of the surface closer together they do not 

 resist it. 



In the oil drops mixed with soot the bending of projections in the 

 manner described is often to be observed under the appropriate condi- 

 tions. They should be compared with the bending of pseudopodia as 



* Similar forms of fluids have been produced by Rhumbler in a parallel man- 

 ner in his imitations of the formation of DifHugia shells (1898, p. 287) and of 

 the shapes of the shells of Foraminifera (1902, p. 265). 



