50 ORGANISMS OF ONE CELL 



coalescence continues until the new vacuole becomes large 

 enough to be seen with low magnification. As it grows by the 

 continual addition of fluids it becomes heavier and less easily 

 carried in the moving protoplasm so that it becomes left behind 

 so to speak until it bursts to the outside, usually in that region 

 which for the time being is posterior. 



Irritability. As a spark may cause an explosion so may cer- 

 tain agents in the environment produce sudden and violent 

 reactions on the part of a living organism. Such reactions are 

 due to the local expenditure of energy. In higher animals 

 special "sensory" organs are activated by heat, light, sound, 

 electrical or other agents called stimuli. The energy released as 

 a response to such stimuli is far in excess of the energy repre- 

 sented by the activating agents and may involve reactions by 

 every part of the organism. 



In Amoeba proteus there are no sense organs but the organism 

 has the property of reacting to every marked change in external 

 conditions in its environment. This property is called irrita- 

 bility and is analogous to more complicated reactions to stimuli 

 in higher animals. Innumerable kinds of stimuli may produce 

 these reactions but may be classified according to their qualities 

 into a few large groups such as mechanical, chemical, photic and 

 electrical all of which indicate changes in the immediate environ- 

 ment of the organism. The responses of the organism to the 

 great variety of stimuli are so diverse that only the most general 

 definition will cover them all. The physiologist Verworn gives 

 such a definition as follows: " Irritability of living substance is 

 its capacity of reacting to changes in its environment by changes 

 in the equilibrium of its matter and its energy" (Lee, Transla- 

 tion,^. 353). 



With Amoeba proteus irritability is indicated by more rapid 

 movement, as under the stimulation of increased temperature; 

 or withdrawing of pseudopodia and rounding out of the body, 

 as under the effects of mechanical, electrical or chemical stimuli. 

 These various responses frequently subserve a useful purpose in 

 capturing food or avoiding difficulties and represent a prototype 

 of higher conscious actions. There is absolutely no ground for 



