THE MOVEMENTS AND REACTIONS OF AMCEBA. 205 



In the foregoing investigation it has been shown that the movements 

 in Amoeba are not of the character supposed by Butschli and Rhumbler. 

 There is, indeed, very little resemblance between the movements of 

 Amoeba and those of an inorganic drop moving as a result of a local 

 change in surface tension. The difference is clearly brought out by a 

 comparison of Fig. 58, showing the currents in Amoeba, with Fig. 34, 

 showing those in the inorganic drop. The more striking differences 

 are as follows : 



(1) In the drop moving as a result of a local change in surface ten- 

 sion the currents on the surface are (and must be) away from the side 

 on which a projection is formed and toward which the drop is moving ; 

 in the Amoeba the surface current is toward this side. 



(2) In the drop the surface currents are in a direction opposed to 

 that of the axial current ; in Amoeba surface currents and axial current 

 are in the same direction. 



(3) The movement of Amoeba is in the nature of rolling, the upper 

 surface passing continually around the anterior end and becoming the 

 lower surface. In the inorganic drop there is no such rolling move- 

 ment, but the interior portions of the fluid are continually passing to 

 the surface at the anterior end. 



Clearly, then, the forces producing the movements in the two cases 

 are not acting in the same manner. The locomotion of Amoeba is 

 demonstrably not due to a local decrease in surface tension on the side 

 toward which the animal is moving. 



This becomes still clearer when we consider in detail the method by 

 which the movements are produced in a drop of inorganic fluid as a 

 result of a local decrease in its surface tension. 



The phenomena of surface tension are usually considered to be the 

 result of the uncompensated attractions of those particles of the fluid 

 which are near to the surface. Such particles are attracted inward and 

 laterally, but not outward (or less strongly outward). The resulting 

 forces may be considered as resolvable into two components, one acting 

 tangent to the surface, the other acting perpendicular to the surface. 

 The former is what may be called surface tension proper ; the latter 

 is often spoken of as normal pressure. The result is very much as if 

 the fluid were covered with a stretched India rubber membrane. 

 These relations are well set forth in a recent paper of Jensen (1901). 

 It is further to be noted that these two components, surface tension and 

 normal pressure, are two aspects of one and the same thing, and, there- 

 fore, vary together and from the same causes ; they can not be separated 

 either theoretically or experimentally. Whenever one of these factors 

 increases or decreases, the other shows a corresponding change. The 

 two are often spoken of (together with the pressure due to curvature of 

 the surface) as surface tension (see Jensen, /. c,). 



