AMEBOID MOVEMENT 71 



tachment to the substratum by very short pseudopods, the surface 

 layer is continually moving toward the anterior end on practically 

 all sides. Every time, therefore, that the ameba moves a little 

 less than its own length, there would accumulate at the tip of the 

 ameba, if it were not removed, an amount of surface layer 

 equivalent to that which covers the whole ameba. No such ac- 

 cumulation can be detected however, from which we infer that 

 it is removed as fast as brought there. And the posterior region 

 of the ameba, which is the main source of the surface film, does 

 not become poorer in this material by reason of its continual 

 flow forward, but new surface is made continually to take the 

 place of that moving forward. This process of destruction and 

 creation of surface is accordingly rapid during active locomotion ; 

 a discoid es, moving approximately once its length at room tem- 

 perature in two minutes, destroying therefore the equivalent of 

 its entire coat of surface in that time; while a sphaeronucleosus, 

 moving once its length in two or three minutes, destroys all its 

 surface every minute. 



From what has been said thus far, it must be apparent that 

 there is striking resemblance between the general movement of 

 the surface layer of the ameba, and of a surface tension layer in 

 a drop -of fluid in which the tension is changed at some point. 

 Let us now inquire briefly into this resemblance. 



As is well known the surface of a liquid in contact with another 

 liquid, solid or gas, with which it does not mix, behaves like a 

 stretched membrane, so that when the tension is reduced at any 

 point the surface layer moves away from that point. A good 

 illustration of the effect of a decrease of surface tension is found 

 in a drop of clove or other oil with which some substance that 

 reduces the surface tension, such as alcohol or soap, is brought 

 into contact at one side. If previously some dust particles have 

 been placed on the surface of the oil drop, it will be easy to see 

 that the surface of the oil moves to the opposite side from where 

 the alcohol or soap solution touched the oil. In practice it is a 

 very simple matter to lower the surface tension of a drop of fluid 

 as described, so as to show the movement of particles on the sur- 

 face. Almost any liquid may be used for this purpose. But it is 

 comparatively very difficult to increase the surface tension at some 



