THE BEHAVIOR OF PROTOZOA 67 



surface of the hinder half of the animal may undergo little 

 change of position for a long time. 



There are various ways of imitating the movements of 

 Amoeba by drops of oil or other fluids subjected to changes 

 of surface tension. If a drop of mercury is placed in dilute 

 nitric acid and a piece of potassium bichromate placed near 

 it the drop of mercury will bulge out toward the bichromate 

 and may surround it. The bichromate as it diffuses against 

 the mercury causes a diminution of surface tension at the 

 region of contact. The stronger contraction of the rest of 

 the surface film forces the mercury to protude at the weakest 

 point, producing an out-pushing resembling the pseudopod 

 of the Amoeba. It has been contended that variations in 

 surface tension account in great measure for the movements 

 of Amoeba and other Rhizopods much as in inorganic fluids. 

 There is certainly a striking analogy between the phenomena 

 in the two cases, but the studies of Jennings have shown 

 that explanation of the phenomena is not quite so simple. 

 Jennings argues that the currents hi Amoeba are not like 

 those occurring in drops of fluid which are changing the 

 surface tension because in the Amoeba there are no lateral 

 return currents which are present in the moving fluid and 

 hence the surface tension theory cannot account for the 

 Amoeba's changes of form. It must not be forgotten that 

 we are dealing with substances of quite different consistency 

 one of which has a cortical layer of considerable thickness and 

 density which is entirely absent in the other, and that the 

 behavior of internal currents may be affected by this factor; 

 but aside from this, there are many facts which seem to in- 

 dicate that the ectoplasm behaves more like a muscular layer 

 than one whose contraction is entirely due to the surface film. 

 Contracting pseudopods may become wrinkled, which could 

 not occur if mere surface film contraction were responsible 



