9/6 ULTRASTABILITY IN THE ORGANISM 



A simple circuit, if excited, would tend either to sink back to 

 zero excitation, if the amplification-factor was less than unity, 

 or to rise to maximal excitation if it was greater than unity. 

 Such a circuit tends to maintain only two degrees of activity: 

 the inactive and the maximal. Its activity will therefore be of 

 step-function form if the time taken by the chain to build up to 

 maximal excitation can be neglected. Its critical states would 

 be the smallest excitation capable of raising it to full activity, 

 and the smallest inhibition capable of stopping it. McCulloch 

 has referred to such circuits as ' endromes ' and has studied some 

 of their properties. 



9/6. Another source of step-functions would be provided if 

 neurons were amoeboid, so that their processes could make or 

 break contact with other cells. 



That nerve-cells are amoeboid in tissue-culture has been known 

 since the first observations of Harrison. When nerve-tissue from 

 chick-embryo is grown in clotted plasma, filaments grow outwards 

 at about 0-05 mm. per hour. The filament terminates in an 

 expanded end, about 15 X 25/^ in size, which is actively amoeboid, 

 continually throwing out processes as though exploring the 

 medium around. Levi studied tissue-cultures by micro-dissection, 

 so that individual cells could be stimulated. He found 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. So the hypothesis that neurons are 

 amoeboid assumes only that they have never lost their original 

 property. It is possible, therefore, that step-functions are pro- 

 vided in this way. 



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