STEP-FUNCTIONS IN THE LIVING ORGANISM 10/5 



that a nerve-cell, touched with the needle-point, would sometimes 

 throw out processes by amoeboid movement. 



The conditions of tissue-culture are somewhat abnormal, and 

 artefacts are common ; but this objection cannot be raised against 

 the work of Speidel, who observed nerve-fibres growing into the 

 living tadpole's tail. The ends of the fibres, like those in the 

 tissue-culture, were actively amoeboid. Later he observed the 

 effects of metrazol in the same way : there occurred an active 

 retraction and, later, re-extension. More recently Carey and 

 others have studied the motor end-plate. They found that it, 

 too, is amoeboid, for it contracted to a ball after physical injury. 



To react to a stimulus by amoeboid movement is perhaps the 

 most ancient of reactions. Reasons have been given in S. 9/1 

 and 9/4 suggesting that adaptation by step-functions is as old as 

 protoplasm itself. So the hypothesis that neurons are amoeboid 

 assumes only that they have never lost their original property. 

 It seems possible, therefore, that step-functions might be provided 

 in this way. 



10/4. A variable which can take only the two values ' all ' or 

 4 nothing ' obviously provides a step-function. It may not always 

 conform to the definition of a step-function, for its change is not 

 always sustained ; but such variables may well provide changes 



B 



w -Time-*- - Time — 



Figure 10/4/1. 



which appear elsewhere in step-function form. Such would hap- 

 pen, for instance, if the change of the variable X (Figure 10/4/1 A) 

 resulted in some accumulative change Y, which would vary as in 

 B. Variables like X could therefore readily yield step-functions. 



10/5. Step-functions could also be provided by groups of neurons 

 acting as a whole. 



Lorente de No has provided abundant histological evidence that 



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