AMEBOID MOVEMENT 81 



face of sphaeronucleosus and of verrucosa in locomotion was 

 found to move from three to three and a half times as fast as 

 the ameba (Chapter VII). In discussing movement in "verrucosa 

 and its relatives" Jennings says "the essential features of the 

 movement seem to be (i) the advance of a wave from the upper 

 surface at the anterior edge; (2) the pull exercised by this 

 wave on the remainder of the upper surface of the body, bringing 

 it forward. Most of the other phenomena follow as consequences 

 of these two" (p. 146). Thus the amount of stretch of the upper 

 surface would exceed the amount of pull on it from 50^ to 75% ! 

 Jennings' explanation of ameboid movement in which the im- 

 portant factor is a more or less permanent ectoplasm in which 

 the ameba rolls along, would unquestionably produce rotary cur- 

 rents. Rhumbler ('98) recognized this and after full considera- 

 tion rejected the idea that the ectoplasm is a permanent skin in 

 which the ameba rolls along in locomotion, because rotary cur- 

 rents are not observed in a moving ameba. Anyone who doubts 

 that rotary currents would be produced under these conditions 

 can convince himself by putting a quantity of glycerine and some 

 shavings in a large transparent rubber balloon or celloidin bag 

 and letting it roll slowly down an incline in front of a strong 

 light. If not too much glycerine is placed in the balloon, the 

 shape of an ameba is closely enough approximated, and the rotary 

 currents down at the posterior and up at the anterior end 

 are well shown. 



From all these considerations it is quite clear that Jennings' 

 explanation of ameboid movement as a rolling movement can 

 not any longer be maintained. His "discussion of this matter 

 (the rolling movement hypothesis) is an excellent example of 

 the fact that acumen and excellent reasoning may lead one astray 

 in scientific matters when the observational basis for the reason- 

 ing is not secure." (These are Jennings' own words in criticism 

 of Rhumbler on the same subject!) 



The surface tension theory, with its many modifications, has 

 had a great many more adherents than any other theory that 

 has been advanced to explain ameboid movement. It represents 

 the attempt of biologists to explain a vital phenomenon on physical 

 grounds. The fact that it has been held to go further in this 



