2 THE ACTION OF THE LIVING CELL 



the nature of the latter. Many more attach themselves 

 to common glass than to quartz, pyrex glass, or paraf- 

 fin. 



If on the other hand any one of fourteen salts is 

 added to the water, the animals attach at once, and 

 become lobose in a few minutes. These experiments 

 show definitely that the environment determines the 

 activity of this unicellular organism. 



Every modification of an organism's activity may 

 be regarded as a response to an external stimulus, — 

 that is, to a change in the physical or chemical nature 

 of the environment. While it is true that one portion 

 of the organism may respond to some change in an- 

 other portion of itself, in the last analysis this primary 

 change in the latter is initiated by variations in the en- 

 vironment. 



The response to external stimuli may be very slow 

 indeed, or may take place with almost explosive ra- 

 pidity. In the latter case the initial stimulus may be 

 regarded as a sort of trigger which releases a rapid 

 series of internal changes, in much the same fashion 

 as the impact of a firing-pin on the primer cap of a 

 high explosive shell initiates a destructive explosion at 

 a considerable distance from the gunner. Indeed, this 

 somewhat crude analogy bears a close resemblance to 

 the chain of events which takes place in a biological 

 system, wherein, as we shall see, a simple physical 

 stimulus often initiates a chemical reaction which in 

 turn propagates a series of vital changes. 



It has been pointed out above that the application 

 of a stimulus to one portion of an organism may pro- 

 duce a response in a distal portion of the same organ- 

 ism. In higher organisms, the conduction of the series 



