ORIGIN OF NEURON PATTERN 189 



As the distance between the axon tip and the regions 

 of high posit ivity in the chief body gradients increases, 

 the positivity of the tip, and therefore its rate of growth, 

 may decrease. In the connection of the axon with its 

 peripheral organ other than electrical factors may per- 

 haps be concerned. Chemical factors, for example, 

 may play a part in some cases. It may be pointed out, 

 however, that if the peripheral organ represents a region 

 of relatively intense growth or functional activity, and 

 therefore of high positivity internally, as the axon comes 

 into its vicinity, it may bring about electrical polariza- 

 tion of the second type in the axon tip and so determine 

 growth of the axon toward itself. It appears probable 

 that the branching of peripheral nerves and the periph- 

 eral distribution and connections of axons with their 

 end organs may be determined, in part at least, in 

 this way through the developmental activity of par- 

 ticular organs, e.g., particular muscles, at the time 

 when particular axon tips grow into their electrical 

 field. 



The growth inward of the axons of peripheral 

 receptors in the invertebrates and in the olfactory 

 primodium of the vertebrates is apparently a reaction of 

 the first type (Fig. 58), like that of the axons in the 

 neural tube, that is, it represents a growth directed by 

 electrical polarization away from the region of high 

 positivity just beneath the surface of the body. As 

 the axons of peripheral receptors approach the central 

 nervous system a polarization of the second type may 

 arise with respect to the negative surface of the central 

 nervous system and so determine their growth toward 

 it. Inside the central nervous system the factors 



