UNICELLULAR FORMS 



63 



limited. When attacked, an amoeba may do only one of three 

 things: It may move, it may become quiet, or it may be unrespon- 

 sive. Responses are o£ necessity slow. It does respond, however, to 

 a variety of conditions. It furthermore is able to select from those 

 available the environment most suitable for its existence. This can 

 be shown by a simple experiment. A culture of amoebae is placed 



:^^^ 



Fig. 15. — Diagram of an amoeba in motion, to show how the cytoplasm changes 

 position and condition with the advance of the leading pseudopodia. The materials 

 in the fluid central region flow forward and are added to the stiffened outer rind. 

 (Simplified after Mast.) 



in a long dish so arranged that the temperature, acidity, or other 

 condition, varies along the dish, forming an intensity or concentra- 

 tion gradient from one end of the dish to the other. In time it will 



Fig. 16. — An amoeba in the process of binary fission. The nucleus has already 

 divided and a constriction is about to separate the animal into two daughter 

 animals. 



be found that the greatest number of amoebse have accumulated 

 in that region of the water that is most suitable for the normal life 

 of the animal. 



