CRITICISM OF ENGELMANN 277 



place. Now if we consider that many simple Amcebce move 

 forward for long distances at a time in exactly the same 

 way that a pseudopodium is protruded, i.e. that they 

 really represent a single creeping pseudopodium, this fact is 

 still more definite evidence than the case upon which we 

 based our argument above, against the complete applica- 

 bility of Engelmann's explanation. There is also the 

 further fact, that while it explains the stream of protoplasm 

 forwards into the pseudopodium, it does not account for the 

 lateral back currents at the ends of the pseudopodium. 



Engelmann believes, however, that the formation of 

 pseudopodia is produced by other causes still. He is, with 

 de Bary, of the opinion that pseudopodia can be pressed 

 forwards, and streaming movements set up, also by a vis It 

 tergo, which depends on local contractions. Finally, he does 

 not wish to refer the origin of the fine filamentous pseudo- 

 podia to contraction, but on the contrary to relaxation of 

 contracted rows of inotagmas. The latter explanation, the 

 mechanical representation of which in itself presents very 

 great difficulties, if one thinks of the considerable length 

 to which pseudopodia of this kind frequently attain, may 

 also be rejected for the reason, that it is improbable in the 

 highest degree that the development of pseudopodia, in 

 which very gradual transitions can be traced so beautifully 

 in the Rhizopod series, should owe its origin to two com- 

 pletely opposite causes. 



Just as unsatisfactory as the explanation of the forma- 

 tion of pseudopodia it seems to me is Engelmann's view as 

 to the causes that produce rotational currents in plant cells. 

 He says on this point (p. 378): "A current of this kind 

 must be brought about when the inotagmas of the layers in 

 motion are, on the whole, orientated with their longitudinal 

 axes parallel to the direction of movement, and the spontan- 

 eous stimulation continues to move forward in tliis direction. 

 The motile protoplasm then creeps upon the non-motile 

 layer of the wall just as the foot of a snail upon the 

 substance beneath it." 



In opposition to this explanation it may be urged that, 

 in the first place, there is certainly a continuous connection 



