58 BIOLOGY 



Reaction to light (Phototropism) (Gr. photos = light + trope). 

 If a strong light is directed upon an Amoeba from one side, it 

 will move away from the light. A strong, white light may ca^se 

 the animal to stop moving. 



Reaction to electricity (Electropism) (Eng. electro + Gr. trope) . 

 If an electric current is passed through an Amoeba, it contracts 

 on the side of the positive pole of the current and moves toward 

 the negative pole. 



In all these cases the Amoeba reacts to a stimulus. But there 

 are other things which are irritable and react to a stimulus in 

 a purely mechanical fashion. Gunpowder is also irritable, since 

 it will react to heat with an explosion. A locomotive is irri- 

 table, since it will react to a touch upon its throttle valve. The 

 Amoeba certainly reacts in a more complex and more variec 

 manner, but the question inevitably arises whether the actior 

 may not be simply that of a bit of machinery responding to its 

 appropriate stimulus. There is no definite answer to this ques- 

 tion that can yet be given. 



Reproduction. As the Amoeba by assimilation converts its 

 food into new protoplasm, it inevitably increases in size. I; 

 this went on without interruption there would be no limit t( 

 the size of the animal. But after growing for a time, a constric 

 tion appears in the middle of the bodywhich deepens until ii 

 finally divides the animal into two parts; Fig. 19 C. Each o; 

 the resulting parts is like the other and each like the original 

 except in size. It is the nucleus that seems to take the lead ir 

 this process of division, which is one of great complexity. Thi; 

 will be described in the next chapter, for it goes through th< 

 complicated series of changes known as karyokinesis (Gr 

 karyon = nucleus -}- kinesis = movement) described on page 85 

 As a result of this division there arise two animals, evidently 

 alike, each of which now moves away and lives an independen 

 life. This method of reproduction, by which the animal divide: 

 into two practically equal parts, is called fission. 



A second method of reproduction sometimes occurs ii 



