THE MOVEMENTS AND REACTIONS OF AMCEBA. 211 



there is a sudden rush forward of the internal fluid from this region, 

 giving the movement a jerky character. 



Still another resemblance in detail between the movements of the 

 inorganic drop and of Amoeba may be noted at times. As we have 

 seen, in the posterior part of Amoeba that is detached from the bottom 

 there is a movement forward not only on the upper surface, but also a 

 slow movement on the lower surface ; the entire posterior region is 

 contracting. The same thing may be seen in the inorganic drop. The 

 phenomenon in question is not so regular here, because the posterior 

 half usually still clings to the surface to a certain extent, while in Amoeba 

 it is as a rule entirely free. But when the posterior half of the inor- 

 ganic drop does become entirely free, it is seen to contract as a whole, 

 with a forward movement on both upper and lower surfaces, exactly 

 as in Amoeba. 



One may even see at times, under special conditions, a slight turning 

 backward of the current at the sides of the anterior end, such as has 

 been described by a number of authors for Amoeba (see p. 137). This 

 occurs when the drop is slender and elongated, and the area on which 

 it spreads out is broad. On coming in contact with the area, the end 

 of the drop rushes forward and spreads out. If the whole width of the 

 area is not covered at first, some of the particles that have moved for- 

 ward curve outward and a little backward till the area is quite covered. 



Altogether, the resemblance between the movements of the inor- 

 ganic drop and those of Amoeba is extraordinary, extending even to 

 details. What are the forces at work in such a drop, and in how far 

 may they be supposed to be active also in Amoeba? 



The spreading out of the drop of glycerine or water at the anterior 

 end is due to its adherence here to the substratum. The remainder of 

 the movements of the inorganic drop are due to the interplay of surface 

 tension and adhesion to the substratum. As a result of surface tension 

 the drop seeks to regain its spherical form ; hence the posterior part is 

 pulled forward, the force required to accomplish this being less than 

 would be demanded for freeing the anterior edge from the substratum. 

 In the pulling forward of the posterior portion the adherence of the 

 lower surface to the bottom keeps this surface from moving ; hence the 

 upper surface moves forward while the lower surface remains quiet 

 or moves forward only very slowly ; the movement is thus converted 

 into a rolling motion. The details given above depend merely upon 

 the relative part played by adherence and surface tension, with the 

 resistance offered by the weight and inertia of particles inclosed in the 

 drop. 



In Amoeba, so far as the evidence of observation goes, the conditions 

 are similar. Amoeba adheres to the substratum and spreads out in a 



