120 RESPONSES OF CELLS ON THE BIOLOGICAL LEVEL 



range of compounds. But more often the response may 

 be evoked by a wide variety of substances and indeed 

 by a wide variety of types of stimuli. For example, the 

 characteristic response of nerve or muscle may be elicited 

 by heat, cold, pressure, cutting, potassium, various or- 

 ganic bases and by electrical shocks. Similarly, the 

 complex series of cellular events resulting in vesication 

 may be elicited by heat, cold, friction, bacterial toxins, 

 arsenoxides and /3-chloroethylamines. It is very tempting 

 to conclude that all the agents producing a given effect 

 must be acting upon the same mechanism, and that by 

 contemplation of what system would have the ability to 

 respond to all of these agents it will be possible to deduce 

 the nature of the reactive mechanism in the cell. Howev- 

 er, more careful analysis of the biological facts frequently 

 indicates that the different agents acting upon a cell may 

 be reacting with quite different systems, although there 

 may be a great deal in common in the final results of 

 such action. For example, a muscle may be stimulated to 

 contract both by treatment with potassium chloride and 

 by treatment with acetyl choline: but the quantities of 

 these substances which are required to produce con- 

 traction are so remarkably different that it is impossible 

 to credit that they act upon the same system. Then again, 

 when the details of vesication are considered , one finds that 

 there are marked differences between the vesicles pro- 

 duced by different agents. Plate II (pages ii6 and 117) 

 shows three types of vesicle, produced by three different 



