METABOLIC GRADIENTS IN AMOEBA 77 



cause fluid drops to change their shape and move about in amoe- 

 boid manner, such alterations of the surface tension due either to 

 internal or external factors are the direct cause of the locomotion 

 of amoeboid organisms. 



Clearly this explanation of amoeboid motion is valid only if 

 the protoplasm is fluid, for although the surface tension of 

 solids is indeed very great, it plays no role in their behavior. 

 It becomes incumbent, therefore, upon the supporters of a surface 

 tension theory to demonstrate that the protoplasm is fluid. This 

 Rhumbler has attempted in a number of papers ('98, '02, '03, 

 and especially '14), where he endeavors to show experimentally 

 that protoplasm is a fluid because it obeys the laws of fluids, 

 namely, it is incompressible, inelastic, and exhibits the phe- 

 nomena of capillarity. I do not wish to take the space here to 

 criticize R humble r's argument adequately but it is certainly 

 not convincing in many respects. My chief objections are: 

 first, that he frequently uses material which can scarcely be re- 

 garded as representative protoplasm, as the yolk of avian and 

 amphibian eggs, and, second, that many of his proofs concern 

 the interior of the protoplasm (streaming movements, presence 

 of spherical inclusions, etc.). That isolated pieces of proto- 

 plasm assume the spherical form is not necessarily a proof of 

 its fluid condition, since the formation of a contractile membrane 

 on the surface would explain the result (as Rhumbler himself 

 remarks), and further the injury of cutting may completely alter 

 the physical state of the protoplasm, probably in the direction of 

 liquefaction. 10 



The surface tension theory plainly demands that the surface of 

 the protoplasm shall be in the fluid state; the condition of the 

 interior is of no moment. Now as a matter of fact when Rhumb- 

 ler attempted to demonstrate that the surface of protoplasm 

 obeys the laws of fluids he met with failure. An amoeba will not 

 spread on the surface of water, as drops of yolk will, although if 

 fluid it must do so, since the surface tension of protoplasm is less 

 than that of water. If a fluid drop suspended in another fluid is 



10 Cf. Chambers ('17). 



