208 THE BEHAVIOR OF LOWER ORGANISMS. 



BERTHOLD'S THEORY THAT ONE-SIDED ADHERENCE TO THE SUB- 

 STRATUM IS THE CAUSE OF LOCOMOTION. 



If, then, the movements of Amoeba are not due to local decrease in sur- 

 face tension, with the formation of "Ausbreitungscentren " (Biitschli), 

 is it possible to find a physical explanation for them ? In taking up 

 this question we must consider separately (i) locomotion, and (2) the 

 formation of pseudopodia. 



Observation and experiment indicate, as we have seen in the obser- 

 vational portion of this paper, that Amoeba is a drop of fluid which 

 becomes attached to the substratum in front and pulls itself forward, 

 the pull extending backward from the attached region over the upper 

 surface, and producing a rolling motion. 



Now, a drop of inorganic fluid under the influence of similar 

 forces moves in precisely the same manner. There is no great diffi- 

 culty in causing a drop of inorganic fluid to adhere more strongly to 

 the substratum on one side than elsewhere. When this is brought 

 about the drop moves toward the more adherent side by a rolling 

 motion, precisely like that of Amoeba. By a proper arrangement of 

 the conditions almost every detail of amoeboid locomotion may be 

 closely imitated. 



That this is the method of movement in Amoeba was the theory 

 maintained by Berthold (1886), though it is rather curious that the 

 supposed facts on which he based this view were incorrect. Berthold 

 confirmed on the basis of observations on Amo&ba verrucosa ( ! ) and 

 other species the account of the currents in Amoeba given by Schulze 

 (see p. 137 and Fig. 37) ; that is, such currents as would be consistent 

 with the theory of local decrease in surface tension, but are quite incon- 

 sistent with his own theory. He rejected the theory that locomotion is 

 due to a decrease in surface tension at the anterior end, on the ground 

 that no currents are to be observed in the surrounding water, as this 

 theory demands. Berthold held that the locomotion is due to the 

 spreading out of the anterior end of the fluid mass on the surface of a 

 solid, this spreading out being due to adhesion between the fluid and 

 the solid. Unfortunately for the understanding of his theory, he tried 

 to bring this into relation with many other much less simple phenom- 

 ena. In particular he compared the movements to those of a drop of 

 water on a glass plate, which flees when a rod wet with ether is brought 

 near one side. This was an unfortunate comparison, as the movements 

 in a drop of water under such circumstances are of a character entirely 

 different from those produced when a mass of fluid adheres by one side 

 to a solid. The movement in a drop of water fleeing from the ether- 

 ized rod is a result of the currents produced by the lowering of the 



