THE MECHANICS OF AMOEBOID MOVEMENT 277 



into threads, and its general lack of resistance to mechanical agencies 

 including surface-tension all afford evidence of its liquid consistency. The 

 viscosity of the protoplasm, varies, however, apart from the direct physical 

 action of temperature upon it l , but it is usually viscous enough to prevent 

 the vibratory molecular movement of small particles embedded in it 2 , 

 and to stop the locomotion of ingested bacteria and Volvocineae 3 , whereas 

 both these movements continue in the cell-sap. The viscous nature of the 

 protoplasm renders the removal of protoplasmic aggregations slow in the 

 absence of streaming movement, or even in its presence 4 , and also results 

 in the plasmolysed protoplasts of elongated cells breaking up less readily 

 into fragments as compared with similar threads of water. External 

 agencies may, however, increase or decrease the viscosity of the proto- 

 plasm either by a direct or indirect action 5 . It may ultimately be possible 

 to determine the actual viscosity of the protoplasm in streaming cells, 

 by measuring the amount of slip of minute oil particles of known 

 density under the action of gravity 6 . 



The same physical factors which regulate the spread of liquids over 

 solid surfaces, and the creeping movements in emulsions of oil and soap_ 

 solution, are also involved in determining the shape and movement of the 

 more or less viscous protoplasm 7 . Thus the spherical shape assumed by 

 a plasmolysed protoplast floating in a liquid of the same density is due to 

 its homogenous surface-tension. If the protoplast is very small it will assume 

 an approximately spherical shape even when resting on a solid substratum 

 just as do sufficiently small drops of mercury, and also drops of water while 

 in the spheroidal condition on a white-hot plate. A local decrease of surface- 

 tension will cause a prominence to appear at that point, and this will 

 continue until the lesser radius of the protrusion enables it to exercise the 

 same centrally directed pressure, as does the larger sphere with a higher 

 surface-tension. The same applies when a distended balloon bulges out at 

 a weak point. If the position of equilibrium is passed and the difference of 

 surface-tension is maintained, the entire mass will be pressed towards the side 

 of least surface-tension, and the impelling force will automatically increase 

 as the radius of curvature of the original body lessens. Drops of a non- 

 miscible fluid lying in a medium or on a substratum may be caused to 

 assume all varieties of form by appropriate local modifications of surface- 

 tension. The spreading movements of a drop of oil upon an alkaline 



1 Ewart, Protoplasmic Streaming in Plants, 1903, pp. 20, 59. 



a Vellen, Flora, 1873, p. 120. 



3 Celakovsky, Flora, 1892, Ergzbd., p. 223. * Ewart, I.e., p. 9. 



6 Ewart, I.e., pp. 10-20, 36, 38. 6 Id., p. 23. 



T For details see textbooks of Physics, and Lehmann, Molecularphysik, 1888, Bd. i, p. 251. 

 Also Berthold, 1. c. ; Biitschli, 1. c. ; Rhumbler, 1. c. ; Jensen, Pfluger's Archiv f. Physiologic, 1901, 

 Bd. LXXXVII, p. 366, and the works of Quincke here quoted. 



