EXPLANATION OF THE MOVEMENTS 313 



as has been said, so especially characterises the posterior 

 end, is, judging from the course it takes, somewhat as follows. 

 As has been remarked above, it is very probable that 

 the surface of the body of the Amoeba is formed by a 

 viscid membrane-like layer. The backward flow of the 

 pseudopodia, like the gradual retraction of the hinder end 

 of the AmoebaB that moves forwards in a simple manner, 

 depends, in my opinion, only . upon the cessation of the 

 forward current, and the flowing back of the protoplasm 

 towards other directions. During this process the pseudo- 

 podium or the hinder end diminishes continually in size, 

 and meanwhile the viscid external layer cannot flow 

 together rapidly enough to keep pace with this diminution, 

 in consequence of which it is thrown into folds, which 

 appear as papillose processes. The whole phenomenon gives 

 just the appearance of a viscid, mernbrane-like layer, which 

 shrinks into folds on account of the gradual emptying of its 

 contents. With continued emptying of the contents these 

 folds become more numerous and fine, so that the com- 

 pletely shrunken layer is finally attached in the form of a 

 tuft-like bundle of fine processes. These processes or folds 

 then become closely apposed, and finally stick together. 

 Meanwhile they are removed from the action of the external 

 medium, which, as we supposed, is the cause of the cuticle- 

 like toughness of the superficial layer ; this removal, as well 

 as the influence of the interior of the Amoeba, which now 

 makes itself felt upon the tuft, has the effect of gradually 

 making it become fluid ; and the folds or processes fuse in 

 the above-described manner, whereupon the product of the 

 fusion and the rest of the protoplasm finally flow together. 



That the origin of the tuft is really due to shrinkage 

 of the membrane-like external layer is also probable from 

 the fact that by treating Amoeba proteus with fairly concen- 

 trated XaCl solution (4 per cent), the same papillar 

 processes can at once be produced over the whole surface, 

 accompanied by considerable shrinkage of the Amoeba, and 

 in this case the processes certainly arise by folding of the 

 external layer, as a result of a decrease in the volume of its 

 contents. With 0*02 per cent caustic potash I also saw 



