OBSERVATIONS ON FEEDING OF AMEBA 41 



b}' a considerable number of the members of our laborator5^ It 

 was, however, later unfortunately lost in the process of staining. 



What bearing have these observations on the surface-tension 

 theory? They have a bearing on this theory^ only in so far as it 

 demands acceptance of the idea that the reactions of Ameba are 

 the result of changes in surface tension. 



We have no direct measurements of the surface tension of 

 protoplasm, but on the basis of various indirect measurements and 

 calculations, Czapek ('11, p. 41-43) concludes that it is approx- 

 imately two-thirds that of water. This conclusion is supported 

 by the facts that protoplasm is rich in organic substances, and 

 that such substances in solution generally tend to lower the sur- 

 face tension of water. Thus we would expect the surface tension 

 of protoplasm to be considerably lower than that of water. 



If this is true, it does not seem possible, a priori, that sufficient 

 power could be developed by changes in the surface tension of a 

 mass of protoplasm like an ameba, either to hold and engulf a 

 struggling nematode or to master a powerful rotifer or to cleave 

 a Paramecium. In reference to the last case mentioned we are 

 fortunate in having a method by means of which the question 

 can be put to an experimental test. 



The power required to cleave a Paramecium can be ascertained 

 with a fair degree of accuracy and from this it is possible to ob- 

 tain a conception of the magnitude of the surface tension re- 

 quired in an ameba to perform the operation. This was accom- 

 plished as follows : One end of a glass rod was drawn out so as to 

 produce a fine flexible fiber about 2 cm. long. A drop of culture 

 fluid containing numerous paramecia was mounted under a bi- 

 nocular. The distal end of the glass rod was now pressed down 

 through the drop. This was repeated more or less at random un- 

 til a Paramecium was caught crosswise near its middle under the 

 end of it. Pressure was then applied steadily until the Parame- 

 cium was cut in two or the rod bent through, an angle of more 

 than 45 degrees. 



In this way coarser and finer rods were tested until one was 

 obtained that would cut a majority of the paramecia when bent 

 through 45 degrees. The pressure exerted by this rod was then 



