THE MOVEMENTS AND REACTIONS OF AMCEBA. 213 



that the forces acting upon the two have a similar localization and 

 direction, not necessarily that the forces themselves are identical. This 

 caution is emphasized by the fact that drops moving down an inclined 

 plane as a result of the action of gravity have a similar rolling motion. 

 This is well shown in the drops of glycerine or water on the oiled 

 surface, in the experiments just described. Most (though not all) of 

 the details mentioned above, in which the movements of the inorganic 

 drop resemble those of Amoeba, may be observed also in drops moving 

 under the influence of gravity. The essential difference between the 

 two sets of experiments is that the action of adhesion, pulling the drop 

 in a certain direction in the one case, is replaced by the action of 

 gravity, pulling in the same direction, in the other case. Correspond- 

 ing with this difference is the chief difference to be observed in the 

 movements of the drops under the different conditions. In those mov- 

 ing as a result of greater adhesion on one side, the anterior edge is 

 thin and flat, as in Amoeba, while in those moving from the action of 

 gravity this is not true. 



In Amoeba observation shows that we have the one-sided greater 

 adhesion, and the tendency of the lower surface to cling slightly to the 

 substratum, as in the first set of experiments with the inorganic drops. 

 There remains only something corresponding to the surface tension 

 factor, common to both sets of inorganic experiments, to be accounted 

 for in Amoeba. Since Amoeba acts like a fluid in many respects, there 

 is no a priori reason to deny it surface tension, and nothing further is 

 required to produce locomotion. To this there is, however, one objec- 

 tion. This is found in the roughening and wrinkling of the surface 

 at the posterior end as it contracts, and in the similar roughening of a 

 contracting pseudopodium (pp. 160, 168). This is exactly the opposite 

 of what should take place in a fluid contracting as a result of surface ten- 

 sion. In such a case the primary phenomenon is the decrease in sur- 

 face ; the latter should, therefore, remain perfectly smooth, and as small 

 as possible. Of course, surface tension might be replaced in Amoeba 

 by a specific property of contractility of some sort, having its seat a 

 little beneath the surface. Locomotion would then take place as in 

 the inorganic drop, and the wrinkling of the outer surface would be 

 accounted for. On the other hand, if we can account for the contrac- 

 tility by a known property of fluids, such as surface tension, our expla- 

 nation will, of course, be simpler and more probable. By taking into 

 consideration the apparent fact that the outer layer of Amoeba is partly 

 fluid, partly solid, I believe that such an explanation, accounting for 

 the roughening as well as the contractility, can be given ; this I shall 

 attempt in the next section of this paper (p. 215). 



The formation of projections at the anterior edge or side of the inor- 

 ganic drop, comparable to the formation of pseudopodia in contact with 



