634 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



performed as follows : A piece of smooth cardboard, such as 

 the Bristol board used for drawing, is placed in the bottom of a 

 flat dish and on a certain spot on the cardboard is placed a drop 

 of water. The whole is then covered with bone oil. This soaks 

 into the cardboard, except where the latter is protected by the 

 drop of water. After the board is well soaked in oil the drop 

 of water is removed, leaving the whole surface covered with oil 

 some millimeters deep. Now a drop of water or glycerine, to 

 which has been added some fine soot, is placed on the cardboard 

 under the oil. This drop is allowed to come in contact by one 

 edge with the area which had been protected from the oil. To 

 this area it adheres, the edge in contact spreads out as a thin 

 sheet, and the rest of the drop is pulled over to the area. Its 

 movement is then exactly that typical for a flowing Amoeba, so 

 that Figure 3 would do equally well for a diagram of the move- 

 ments of such a drop as for those of Amoeba. The resemblance 

 extends to minute details ; many of these are set forth in the 

 author's paper above cited (:04). Among other things, the 

 formation of pseudopodia in contact with the substratum may 

 be imitated by making the area to which the drop adheres at one 

 edge very small ; then a projection is formed merely of the width 

 of this area. 



But this imitation, like the others, fails when we take into 

 consideration the formation of pseudopodia which are nowhere 

 in contact with a solid. Projections corresponding to these can- 

 not be formed in the physical experiments just described, for in 

 these adherence to a solid is the essential point. Since the 

 entire anterior end of the Amoeba can be pushed out into the 

 free water, we find that Amceba can perform all the active 

 operations concerned in locomotion without adherence to a 

 solid. This effectually blocks any attempt to explain the move- 

 ments of Amoeba as due, like those of the drops in the exper- 

 iments just described, to one-sided adherence to the substratum. 



Thus none of the physical imitations gives us a clue to the 

 physical agent actually at work in the movements of Amoeba. 

 The experiments last described are perhaps useful in giving us 

 an idea of the direction of action of the forces at work in pro- 

 ducmg locomotion. Not even so much as this can be said of 



