28o THEORETICAL BIOLOGY 



by step, and each step forward that it makes is taken up 

 by it as an indication. 



Since, however, the steps that it takes follow a line pre- 

 sented from without, they act as an objective indication, and 

 can unite with those that are optical into one that is objective, 

 without there being anything left over. 



The diagram for the steering-mechanism of a receptor 



action would look like this :~ 



■^ 



First we see the receptor, which is moved by the receiving 

 effector. The receptor transmits its excitation to the mark- 

 organ, while the receiving effector gets its excitations from 

 the action-organ. These excitations are in part turned off 

 from a central receptor, and sent to the mark-organ, where 

 they arrive in company with the waves of excitation proceed- 

 ing from the receptor. 



In the general formula for the action based on experience, 

 the receptor action does not find expression, because this treats 

 all the receptor effects in the same way. 



THE IMPULSE IN THE NERVOUS SYSTEM 



We have learnt about the invasion of the germ by the 

 impulses, and we know that they affect the genes which lie 

 all together, side by side, in the nucleus of the fertilised egg- 

 cell. But where do the impulses invade the completed animal, 

 when it performs an action ? Even if we restrict the invasion 

 here also to the genes in certain cells, we still have to inquire 

 where these cells are to be looked for. The formulae for the 

 majority of actions refer us to the two central points — the 

 mark-organ and the action-organ. It is here accordingly 

 that we must look for the cells in question. 



To determine on these cells I must first go back to what 

 I said in the section dealing with the theory of indications. 



