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 
Ameeba 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 Amceba. It has been contended that variations in 
surface tension account in great measure for the movements 
of Ameeba 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 in Amceba are not like 
those occurring in drops of fluid which are changing the 
surface tension because in the Amceba there are no lateral 
return currents which are present in the moving fluid and 
hence the surface tension theory cannot account for the 
Amceba’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 
