GENERAL ZOOLOGY 



Fig. 8.3. Amoeba: reactions to contact with sui)stratum. (Adapted from H. S. Jcnnms^s, J /if 

 Behavior of the Lower Organisms, copyright 1906 by Columbia University Press, printed by 

 permission.) 



movement is based on the assumption that a relatively stiff, elastic layer, 

 the plasmagel, surrounds the cell just beneath the plasmalemma and en- 

 closes the more fluid inner contents, or plasmasol (Fig. 8.2). Localized 

 changes cause a temporary liquefaction of the gelatinous outer layer at the 

 point where a pseudopod is to arise; the elasticity of the remainder of the 

 gelatinous sheath forces the fluid endoplasm against and through such a 

 weakened area. Within the pseudopodial lobe thus formed, the fluid endo- 

 plasm flows peripherally and stiffens, adding to the plasmagel layer. This 

 type of movement, therefore, involves one of the fundamental capacities of 

 the endoplasm: that of changing its physical state from gel to sol, and the 

 reversal of this process. An amoeba, we may say, moves as a tunnel might, 

 if the mortar of its wall became fluid at the posterior end and flowed within 

 the tunnel to its anterior end, carrying the bricks to be laid again anteriorly 

 by a new setting of the mortar. 



Just as the collective reflexes make up the behavior of multicellular organ- 

 isms such as vertebrates (see p. 86), the movements and other reactions of 

 an amoeba in response to stimuli, or changes in the environment, make up the 

 behavior of this unicellular animal. We may compare the activities of the 

 single-celled amoeba with those of a white blood cell in the vertebrate body 

 (see p. 63), or we may think of the amoeba as an individual animal to be 

 compared with another complete individual. In the former case, we compare 

 cell with cell, and the parallels are obvious. In the latter comparison, we 

 forget about the cellular organization and think only of the individual as a 

 whole. The behavior of white blood cells, when they move into certain 

 regions and ingest such foreign bodies as bacteria, may be compared with the 

 behavior of amoebas. The behavior of other cells in the vertebrate animal 

 may be similarly compared, but the correspondence is less evident. 



Thoughtful consideration of an amoeba as an individual, reacting to its 

 environment, and in comparison with another individual, such as the many- 

 celled vertebrate, enables us to recognize broad factors common to each, and 

 to state them in general terms, irrespective of the cellular organization of 

 either animal. Nevertheless, what is called the behavior of an amoeba is 



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