THE MOVEMENTS AND REACTIONS OF AMCEBA. 209 



surface tension on one side, as Biitschli (1892, pp. 191 and 194), has 

 shown. This comparison, and Berthold's discussion of the relations 

 between spreading out on the surface of solids and on the surface of 

 liquids, together with his incorrect idea of the currents in such spread- 

 ing out on a solid, have served to distract the attention of investigators 

 from the really simple essential features of such a theory. Berthold 

 did not attempt to study directly the currents and other movements of 

 a drop of fluid moving as a result of one-sided adherence to a solid. 

 We may, therefore, leave his account and examine for ourselves the 

 phenomena in question. 



EXPERIMENTAL IMITATION OF THE LOCOMOTION OF AMCEBA. 



The experiments with inorganic fluids may be performed as follows : 

 A piece of smooth cardboard, such as the Bristol board used for drawing, 

 is placed on the level bottom of a shallow vessel, such as a Petrie dish, 

 and soaked with bone oil by spreading the latter over its surface. A 

 small area on the surface of the board is protected from the oil by 

 placing upon it a drop of water. After the board has become well 

 soaked, the drop of water is removed with a pipette, leaving this spot 

 merely damp, while a layer of oil some millimeters deep is poured into 

 the vessel, covering the cardboard completely. A drop of glycerine or 

 of water is then introduced; this settles to the bottom, but adheres to 

 it only slightly. A drop of glycerine is in some respects preferable, as 

 its movements are slower. To the drop should be added beforehand a 

 quantity of soot, in order to make its internal movements visible. 

 Some of the soot remains on the surface, projecting out into the oil, 

 thus making it possible to observe the surface currents. 



If the drop is brought close to the spot on the cardboard that was 

 protected from the oil, so that one side comes in contact with this 

 region, the edge of the glycerine or water drop spreads out over this 

 area. Thereupon the remainder of the drop is pulled in that direction, 

 till the whole drop takes up its position over the protected spot. In 

 the movement of the drop toward the area to which one side adheres, 

 it rolls exactly as Amoeba does. The currents on the upper surface 

 and within the drop are forward. Toward the sides the currents are 

 somewhat less marked, and on the under surface they cease entirely ; 

 particles within the drop but in contact with the lower surface are not 

 moved at all. The forward current is most rapid in front, becoming 

 slower at the rear, exactly as in Amoeba. At the posterior end the 

 surface rolls upward ; particles on the surface which were at first on 

 the bottom may be seen to pass upward around the posterior end and 

 then forward, as in Amoeba. The form of the drop may become much 

 elongated; the anterior edge is thin, the posterior end thick and 

 rounded. In all these respects the drop resembles the moving Amoaba. 



