28 



AMGEBA. PHYLUM PROTOZOA 



form than is usual in animals. A pseudopodium begins as an out- 

 flowing of the ectoplasm, into which the endoplasm presently 

 flows. The projection grows, and in time so much of the body has 

 moved into it that protoplasm has to be drawn up from behind 

 to take its place, and locomotion has occurred ; this is known as 

 amoeboid movement (Fig. lo). Subsidiary pseudopodia are formed 

 at the side of the main one, but these are eventually withdrawn. 

 It appears that whatever happens to the amoeba, it is always the 



Fig. io. — Successive changes in shape of an individual of Amoeba proteus, 



drawn at intervals of two minutes. 



same end which leads, so that in spite of its apparent irregularity, 

 the animal really has right and left sides. In another species 

 {A. Umax) which forms only one pseudopodium and is therefore 

 easier to observe, it has been shown that the formation of pseudo- 

 podia takes place in the following manner. The ectoplasm and the 

 outer part of the endoplasm together form a firm coat, the plas- 

 magel, around the fluid inner endoplasm or plasmasol. Where 

 a pseudopodium is to be thrust out the plasmagel softens, and 

 the contraction of the rest of that layer then presses the plasmasol 

 towards this spot, which bulges. As the bulge grows, a covering 

 of plasmagel for its flanks is provided by conversion of plasmasol. 



