156 THE BEHAVIOR OF LOWER ORGANISMS. 



examples above given are typical for all. They show the following as 

 to the manner in which the pseudopodia are formed when they are 

 projected freely into the water. 



1. The pseudopodium grows in length chiefly from the base, so that 

 any part on the surface retains nearly its original distance from the tip. 



2. The increase in surface as the pseudopodium grows is not pro- 

 duced by the flowing outward and backward of the endosarc at the tip 

 with its transformation into ectosarc (as represented by Fig. 31), but 

 by the transference of a portion of the surface layer of the body to the 

 pseudopodium. The same substance remains at the tip of the pseu- 

 dopodium from the beginning (observation 2, p. 154 ; I have other 

 observations showing the same thing). 



3. Thus the movement of the free pseudopodium is like that of the 

 pseudopodium in contact with a surface, save that in the latter case one 

 side is held back by attachment to the substratum. In the free pseudo- 

 podium all sides move outward ; in the attached one, all sides but one. 



The outer layer of the body in its transference to the pseudopodium 

 may doubtless become thicker or thinner or be otherwise modified. As 

 will be shown later, I am not at all inclined to deny the possibility of 

 the transformation of endosarc into ectosarc, and vice versa. The 

 observations show, however, that this transformation of substance does 

 not, as a rule, take place in pseudopodia by means of the ''fountain 

 currents" represented in the diagrams from Rhumbler (Figs. 30-32). 



Further, the surface of the pseudopodium may be increased by the 

 flowing into it of the endosarc, producing a sort of stretching of the 

 outer layer, involving, of course, the appearance at the surface of por- 

 tions of substance which were before covered. 



WITHDRAWAL OF PSEUDOPODIA. 



In the withdrawal of pseudopodia the process is the reverse of that 

 occurring in the formation of pseudopodia, as is shown in case 3, above 

 (p. 154, Fig. 49). The basal parts of the pseudopodial surface first 

 pass on to the body, followed by the distal portions. 



The withdrawal of pseudopodia shows certain other features that are 

 of importance for the understanding of the mechanism of amoeboid 

 movement. The process differs somewhat in different cases, depending 

 on whether the withdrawal is slow or rapid. When the pseudopodium is 

 slowly withdrawn, its surface may remain perfectly smooth, the decrease 

 in surface keeping pace with the decrease in volume, until the pseudo- 

 podium has quite disappeared. But when the withdrawal is more rapid 

 the surface becomes thrown into folds or warty prominences of various 

 sorts. This is more common than retraction without the formation of 

 such prominences. Evidently the volume decreases so fast that the 



