THE BIOLOGICAL UNIT 41 



1. The ameba is able to distinguish between 

 different particles of matter. Each particle must 

 then give to the ameba a different kind or form 

 of stimulus. Such facts are considered under the 

 term irritability or the power to appreciate a 

 stimulus. 



2. Such stimuli were obviously conveyed within 

 the periphery of the protoplasm as changes in 

 shape took place in regions separate from the 

 point of the stimulus. This is designated as con- 

 ductivity or the power to carry or convey a stim- 

 ulus. 



3. The observed facts in connection with the 

 ameba's response to different stimuli show that 

 the protoplasm assumes various positions in rela- 

 tion to the foreign body. In short the protoplasm 

 was made to contract — much as a muscle con- 

 tracts; and the term contractility, is used to de- 

 scribe such changes. 



4. But between the appreciation of the stimulus 

 and the contraction of the body some adjustment 

 has taken place within the protoplasm. For the 

 ameba hungry and the ameba that has just eaten 

 its fill act in an entirely different manner when 

 particles of food are met. There are really no 

 impersonal terms which can be employed to de- 

 scribe these conditions, so we have to use those that 

 usually have a human meaning. In doing this, 

 however, we are simply trying to make clear what 



