A MSB A 



67 



if it is forceful. When coming quietly in contact with food or with any 

 indifferent object an ameba tends to increase the amount of body surface 

 in contact, but when anything touches it at all violently it draws back and 

 moves away. It avoids a strong light but does not seek darkness, so it 

 selects an optimum. This reaction may be complicated by the effect of 

 temperature when it is exposed to the rays of the sun. To chemicals in 

 solution the response varies with the character of the chemical. To the 

 normal constituents of the water in which the animal Hves it is indifferent; 



Nucleus 



Fig. 21. — ^Fission and sporulation in Amoeba. Fission is illustrated in A to D, which 

 represent four stages in fission in a European species. {Taken from Doflein, " Lehrbuch der 

 Protozoenkunde," after F. E. Schulze.) E and F show sporulation in Amoeba proteus. 

 (Also from. Doflein, by the courtesy of Gustav Fischer.) 



to substances which indicate the presence of food it responds positively, 

 being thus brought to its food; to substances, however, which are not 

 normal constituents of the water and which tend, therefore, to injure 

 the ameba it responds negatively. Amebas have an optimum at a low 

 temperature. Cold benumbs them as the temperature approaches the 

 freezing point, while a temperature above 30°C. (86°F.) also retards their 

 activities. 



98. Reproduction. — At intervals amebas reproduce, doing so whenever 

 they reach the limit of size, which in Amoeba proteus is about 0.25 mm., 

 or 0.01 inch. 



The ordinary form of reproduction is known as fission, or binary divi- 

 sion. In this process the body elongates and a constriction appears in the 



