64 AMEBOID MOVEMENT 



cates beyond question that it is intimately associated with ameboid 

 movement as it is ordinarily understood in amebas, and that it is 

 almost certainly a "necessary" physical consequence of the more 

 fundamental physical processes involved in the movement of 

 amebas. 



That the third layer moves in the same general direction as 

 the ameba has already been mentioned. The direction of a moving 

 particle is however not necessarily parallel with the stream of 

 endoplasm below. In a retracting pseudopod that lies nearly 

 parallel to and by the side of the main advancing pseudopod, the 

 particles on the far side and near the base frequently move across 

 the pseudopod at an angle (and therefore also across the endo- 

 plasmic stream), and up the active pseudopod on the near side. 

 This shows conclusively that the direction of flowing endoplasm 

 by itself has no direct connection with the direction of flow of the 

 surface layer. 



To say that the particles carried by the surface layer bring up 

 at the anterior ends of pseudopods or of the ameba when in 

 clavate shape, admits of further qualification. The advancing 

 edge is not a straight line but an arc, and the sides near the ad- 

 vancing edge are building at a slower rate than the extreme tip. 

 The most rapid formation of ectoplasm is at that point of the 

 ameba that is farthest ahead. At this point all the ectoplasm 

 to be made is still to be made, but as one passes back along the 

 side of the pseudopod more and more ectoplasm is encountered 

 and less and less remains to be made. There is therefore a 

 gradient in the rate and in the amount of ectoplasm formed as 

 one passes back from the forward end of the longitudinal axis 

 of the pseudopod along the side. This is especially the case with 

 certain amebas like Amoeba discoides, A, laureata and others in 

 which the pseudopods -are more nearly cylindrical. In such 

 amebas as A. proteus and A. verrucosa, the factor of ridge forma- 

 tion complicates to some extent the longitudial gradient of ecto- 

 plasm formation. But in spite of these specific differences, the 

 general statement still holds that the rate of ectoplasm formation 

 at the extreme anterior end is higher than anywhere else in the 

 ameba, and that the rate gradually falls to zero as the nearly 

 straight and parallel sides of the pseudopod or ameba, as the 

 case may be, are approached. 



