IRRITABILITY 



45 



from the object by sending out tiny projections of cytoplasm, 

 which are known as pseudopodia. 



If an amoeba comes in contact with food, it surrounds the 

 particles. If touched with wires carrying an electrical charge, it 

 rolls itself up into a ball as if shocked. It show^s no activity at 

 freezing temperature, and it evidences the greatest activity at a 

 temperature of about 85° F. It wdll become spherical, hard, and 

 lifeless in very hot water. If a beam of light is directed toward 

 one side, the amoeba will move away from the light and toward 

 the darkened portion. If a grain of sugar is placed in a drop 

 of water, the amoeba will move toward the sugar ; if a grain of 

 salt is placed there, it will move away. If, with a very fine 

 pipette, a chemical is injected into the amoeba's cytoplasm, the 

 animal is able to sever off almost immediately that part of the 

 cytoplasm and will move from the rejected part as quickly as 

 possible. Recently, Dr. R. H. Chambers has developed methods 

 of micro-dissection. B}^ means of a microscope and a set of tiny 

 dissection instruments and capillary pipettes he can dissect or 

 inject chemicals in or near specimens while they are mounted under 

 the microscope. 



The responses or activities of the amoeba called forth by stimuli, 

 are tropisms (tropos — a turning) . These responses may be toward 



When the amoeba reaches its maximum size, the nucleus elongates and divides into two nuclei. 

 The animal constricts and breaks apart into two daughter cells, each with one nucleus. 



the stimulus, in which case the tropism is called a positive tropism 

 and designated by a plus sign ( + tropism). If the resulting ac- 

 tivity is a motion away from the stimulus, the tropism is con- 



