AMEBOID MOVEMENT 77 



causal manner. It does not at present give us any greater in- 

 sight into the ultimate cause of ameboid movement, although it 

 is clear that an important step in this direction has been taken. 

 But no theory of ameboid movement can be accepted that de- 

 mands conditions in the ameba that are contrary to those described 

 in the preceding chapters, in connection with the surface layer. 

 From this point of view therefore the discovery of the true nature 

 of the outside surface of the ameba is of importance, for it widens 

 to a very considerable extent the observational basis with which 

 any theory of ameboid movement must conform. Since the prop- 

 erties of the outer layer are here described in detail for the first 

 time, it becomes necessary to enquire to what extent the more com- 

 monly held theories of ameboid movement conform with the ob- 

 served behavior of the surface film. Although the surface tension 

 theory was the first detailed theory proposed toward an explana- 

 tion of ameboid movement, I shall discuss Jennings' ('04) ob- 

 servations on the movements of the ameba first, because a great 

 part of his work deals with the movements of the surface film, 

 although he did not recognize it as distinct from the ectoplasm 

 in its movements. 



It is generally recognized that Dellinger's ('06) work proved 

 that Jennings' conception of the ectoplasm as a permanent skin 

 in which the ameba rolls along, is probably inadequate for such 

 amebas as proteus; though singularly enough it is still supposed 

 that verrncosa and its congeners move in the way described by 

 Jennings (Hyman, '17, p. 83). 



Jennings ('04) describes the movements of amebas, both pro- 

 teus and verrucosa "types," as follows : 



"In an advancing Amoeba substance flows forward on the up- 

 per surface, rolls over at the anterior edge, coming into contact 

 with the substratum, then remaining quiet until the body of the 

 ameba has passed over it. It then moves upward at the posterior 

 end, and forward again on the upper surface, continuing in ro- 

 tation as long as the ameba continues to progress. The motion of 

 the upper surface is congruent with that of the endosarc, the two 

 forming a single stream (p. 148). 



"We have demonstrated above, for Amoeba at least, that the 

 forward movement is not confined to a thin outer layer, but ex- 



