42 AN INTRODUCTION TO ZOOLOGY 



body of the Ameba may move in a certain direction for the same 

 reason (27). This theory, however, accounts only for the 

 pseudopodia that are attached, and does not explain free pseudo- 

 podia. 



(2) The Surface Tension Theory. According to the surface 

 tension theory, local changes in the surface tension produce 

 currents which move forward in the central axis and backward 

 along the surface (28, 44). It has been shown, however, that 

 the currents do not run in this manner (34). 



(3) The Contractile Theory. Many early investigators held 

 the view that the contractility of the protoplasm accounts for 

 changes which take place in a moving Ameba. This theory was 

 later given up by most zoologists because the currents in the pro- 

 toplasm began at the point of advance and extended backward 



FIG. ii. Diagram of the movements of a particle attached to the outer sur- 

 face of Ameba verrucosa, in side view. (From Jennings.) 



instead of commencing at the hinder end or in the center. Two 

 recent authors have again brought forth evidence which tends 

 to show that after all the contraction theory is the correct inter- 

 pretation. Jennings (39) by mixing soot in water was able to 

 show that Ameba verrucosa resembles an elastic sac filled with a 

 fluid. Specimens were placed in water containing some fine soot. 

 The particles became attached to the surface of the animal and 

 the currents in the ectoplasm could easily be determined by 

 watching their movements. From these observations the follow- 

 ing conclusions were reached. "In an advancing Amoeba sub- 

 stance flows forward on the upper surface, rolls over at the ante- 

 rior edge, coming in contact with the substratum, then remains 

 quiet until the body of the Amoeba has passed over it. It ther 



