AMEBOID MOVEMENT 5 



ameba as a whole. He denied the validity of Biitschli's sugges- 

 tion that there might be a thin third layer on the outside of amebas 

 or pelomyxas. 



But the existence of a third layer of protoplasm as distinct from 

 the ectoplasm, was again maintained by Schaeffer ('17) who 

 found that in some amebas the outer surface moves forward 

 faster than the ameba advances through the water. The third 

 layer was found to be generated over the surface of the ameba, 

 especially in the posterior region of the ameba, and destroyed at 

 the anterior end. 



But the purely observational aspect of the problem of ameboid 

 movement has not interested biologists generally as much as the 

 ultimate cause of the phenomenon. 



The first attempt that was made to explain ameboid movement 

 in conformity with the demands of modern experimental science, 

 that is, on the basis of physical factors, was made by Berthold 

 ('86). By means of simple experiments with inert fluids (oils, 

 alcohol, water, ether) which were modeled after an experiment 

 described by the physicist Paalzow ('58), Berthold concluded 

 that locomotion in ameboid organisms is due to the physical at- 

 traction of the anterior end to the substratum. The ameba was 

 supposed to behave like a drop of fluid which moved towards the 

 point where the tension of the ameba's surface was decreased by 

 contact with the substratum. The ameba did not push out pseu- 

 dopods according to Berthold, but they were pulled out because of 

 a difference in surface tension between them and the substratum. 

 But pseudopods which were extended into the water and out of 

 contact with a solid substratum, were said to be extended by a 

 contractile effort of the posterior region of the ameba. 



Biitschli ('92, p. 187) pointed out that it was highly improb- 

 able that pseudopods in contact with a solid substratum were pro- 

 jected in a fundamentally different way from that in which free 

 pseudopods were extended, as explained by Berthold. Biitschli 

 assumed that all ameboid movement was due to the same funda- 

 mental cause. He postulated surface tension as the active agent, 

 as Berthold had done for the extension of pseudopods in contact 

 with a solid substrate; but Biitschli assumed that the decrease in 

 surface tension at the anterior end of the ameba was brought 



