212 THE FORMS OF CELLS [ch. 



we bring into its neighbourhood a heated wire, or a glass rod 

 dipped in ether. When we find that a plasmodium of Aethalium, 

 for instance, creeps towards a damp spot, or towards a warm spot, 

 or towards substances that happen to be nutritious, and again 

 creeps away from solutions of sugar or of salt, we seem to be 

 dealing with phenomena every one of which can be paralleled by 

 ordinary phenomena of surface tension*. Even the soap-bubble 

 itself is imperfectly in equilibrium, for the reason that its film, 

 like the protoplasm of amoeba or Aethalium, is an excessively 

 heterogeneous substance. Its surface tensions vary from point 

 to point, and chemical changes and changes of temperature 

 increase and magnify the variation. The whole surface of the 

 bubble is in constant movement as the concentrated portions of 

 the soapy fluid make their way outwards from the deeper layers ; 

 it thins and it thickens, its colours change, currents are set up in 

 it, and little bubbles glide over it; it continues in this state of 

 constant movement, as its parts strive one with another in all 

 their interactions towards equilibrium f. 



In the case of the naked protoplasmic cell, as the amoeboid 

 phase is emphatically a phase of freedom and activity, of chemical 

 and physiological change, so, on the other hand, is the spherical 

 form indicative of a phase of rest or comparative inactivity. In 

 the one phase we see unequal surface tensions manifested in the 

 creeping movements of the amoeboid body, in the rounding o£E 

 of the ends of the pseudopodia, in the flowing out of its substance 

 over a particle of "food," and in the current-motions in the interior 

 of its mass ; till finally, in the other phase, when internal homo- 

 geneity and equilibrium have been attained and the potential 



* The surface-tension theory of protoplasmic movement has been denied by 

 many. Cf. (e.g.), Jennings, H. S., Contributions to the Study of the Behaviour 

 of the Lower Organisms, Carnegie Inst. 1904, pp. 130-230; Delhnger, O. P., 

 Locomotion of Amoebae, etc. Journ. Exp. Zool. iii, pp. 337-357, 1906; also various 

 papers by Max Heidenhain, in Anatom. Hefte (Merkel und Bonnet), etc. 



"]" These various movements of a liquid surface, and other still more striking 

 movements such as those of a piece of camphor floating on water, were at one time 

 ascribed by certain physicists to a peculiar force, sui generis, the force e'pipolique 

 of Dutrochet : imtil van der Mensbrugghe shewed that differences of surface tension 

 were enough to account for this whole series of phenomena (Sur la tension super- 

 ficielle des hquides consideree au point de vue de certains mouvements observes 

 a leur surface, Mem. Cour. Acad, de Belgique, xxxiv, 1869; cf. Plateau, p. 283). 



