no 



THE RISE OF ANIMAL LIFE 



retractinq pseudopodium 

 plasma membrane 

 ectoplasm 

 endoplasm 



nucleus 

 food vacuole 



controctile vacuole 



pseudopodia 



temporary anterior end 



Fig. 7-3. Amoeba with internal anatomy shown in detail. 



AMOEBA 



The common amoeba, Amoeba proteus 

 (Fig. 7-2), spends its life today, just as it 

 did many millions of years ago, in an 

 aquatic environment from which it receives 

 all of its nourishment and into which it 

 deposits all of its wastes. A food-laden 

 world lies within its ability to apprehend 

 and devour. Its continual search for food 

 keeps it on the move, crawling over vege- 

 tation on the bottom of ponds and streams. 

 It resembles an amorphous blob of nearly 

 transparent jelly and its only outward mani- 

 festation of animal affiliation is its move- 

 ment, which consists of an interesting and 

 bizarre method which seems to be confined 

 almost exclusively to this tiny animal. 



Locomotion 



Amoeba moves about by means of minute 

 projections of its protoplasm, called pseudo- 

 pods (pseudopodia — false feet). They are 

 formed by what at first seems to be a 

 very simple process — the fluid protoplasm 

 merely flows out into an apparently weak- 

 ened region in the outside layer, called the 

 ectoplasm. Further observations indicate 

 that the process is not a simple one. For 

 example, pseudopodial formations seem to 

 be "intentional" because they develop only 

 in order to approach food or to move away 

 from danger. Evidently there is some type 



of coordinating mechanism located within 

 the cell body which makes this possible. 



The first indication of pseudopod forma- 

 tion is activity in the protoplasm at a par- 

 ticular point (Fig. 7-3). The ectoplasm 

 then flows out into this region. Simultane- 

 ously, the temporary posterior end gives 

 up its position, and that protoplasm moves 

 forward, filling the region left by the proto- 

 plasm which is actively forming the new 

 pseudopod. Pseudopods form vertically and 

 anteriorly, that is, in the direction of the 

 cell's general progress. The amoeba is able to 

 "step over" particles which are not food and 

 to engulf those which are a part of its diet. 



The inner portion of the cytoplasm 

 (known as endoplasm) consists of a very 

 fluid inner region called the plasmasol, and 

 a more viscous region just outside the fluid 

 region, the plasmagel. These two may 

 quickly change from one consistency to 

 the other (phase reversal, p. 48) and that 

 is what happens when the amoeba "walks." 

 Precisely, the plasmasol flows in the direc- 

 tion in which the pseudopod is to form, and 

 changes to plasmagel as it spreads out at 

 the tip. The flow of plasmasol is continual 

 as long as the cell moves in that particular 

 direction. Most descriptions of this process 

 point to the fact that the pseudopods form 

 in several directions at one time. There fol- 

 lows an apparent "tug-of-war" until finally 

 the pseudopods on one side or the other 



