THE ORIGIN OF THE NERVOUS SYSTEM 



determining their distribution must be similar to those 

 acting in the case of other axons. In many cases, how- 

 ever, the direction of growth of the axons of peripheral 

 receptors is altered almost immediately after their origin 

 by the presence of axial gradients, which are more 

 effective than the surface-interior gradients in deter- 

 mining polarization and directed response. Even in 

 the slightly differentiated sensory cells of the actinian 

 tentacle the outgrowth apparently reacts almost immedi- 

 ately after its origin to the axial gradient of the tentacle 

 and grows in most cases down this gradient (Fig. 18). 

 Axons may apparently grow up as well as down a 

 general body gradient. According to the polarization 

 hypothesis, such growth may result from polarization 

 of the first type with respect to a region of high positivity 

 in continuity with the axon, which gives rise to a current 

 in the opposite direction from that of the general 

 gradient and sufficiently strong to overcompensate it. 

 In such a case the axon is in reality not growing up a 

 gradient but down the local gradient to which it is 

 reacting. Undoubtedly in the course of development 

 of the central nervous system such local gradients fre- 

 quently arise. For example, the approach of an axon 

 or a group of axons to other cells less advanced in 

 development or the localized activity associated with 

 the development of a body segment may determine 

 such polarization and direction of growth. An axon 

 growing in a local gradient against a general axial 

 gradient can of course grow only so far as its polariza- 

 tion is maintained by the local gradient. Such axons 

 are therefore likely to be short, while those growing 

 down the general gradients are much longer. A very 



