THE MOVEMENTS AND RKACTIOXS OF AMCEBA. 



153 



podia are slender and pointed, the protoplasm flows forward (toward 

 the point) on the free upper surface and in the interior, while on the 

 side which is in contact with the substratum the protoplasm is at rest. 

 I have often seen sinall particles which had been brought forward on 

 the surface of the AnKsba carried out to the tip of a pseudopodium on 

 its upper surfoce, finally rolling over the point and becoming covered 

 by the advancing protoplasm (Fig. 46). 



FORMATION OF FREE PSKUDOPODIA. 



When a pseudopodium is sent out directly into the water, so that its 

 surface is free on all sides, it is much more difficult to determine the 

 nature of the movement. Particles rarely cling to the surface of such 

 a pseudopodium, and without this aid one cannot be certain what the 

 movement of the outer layer is. However, by devoting several entire 

 days under most favorable conditions to the determination of this point, 

 I collected a number of observations which demonstrate clearly the 

 nature of the move- 

 ment. The point \ a ^ ^ 

 of special interest 

 is, here as else- 

 where , whether 

 there is a back- 

 ward current on 

 the surface of the 

 advancing pseudo- 

 podium, as repre- 

 sented in the dia- 



FiG. 47.* 



gram from Rhumbler, Fig.31. To this the observations give a negative 

 answer. Particles clinging to the surface of a pseudopodium, whether 

 at the tip or at the base or at any intermediate point, are uniformly 

 carried outward, in the same direction as the tip. Particles situated at 

 a certain distance from the tip of a short pseudopodium maintain the 

 same distance as a rule when the pseudopodium is lengthened, though 

 in so doing they are carried far out from the body. Sometimes the tip 

 moves outward a little faster than the parts behind it, the pseudopodium 

 thus becoming more slender as it extends, but all parts agree in being 

 carried outward. A number of examples of actual observations will 

 make this point clear. 



I. Amceba angulata : When first observed there was a short pseudo- 

 podium in front, projecting freely into the water. A small particle was 

 attached to the surface at about the middle of its length (Fig. 47, a). 



* Fig. 47. — Successive stages in the formation of a free pseudopodium, showing 

 the movement of a particle attached to its surface. The particle moves outward, 

 keeping at approxitnatelj the same distance from the tip. 



