192 THE ORIGIN OF THE NERVOUS SYSTEM 



plants and colonial animals with a main axis and lateral 

 branches, and since it has been shown that physiological 

 isolation is the primary factor in the development of 

 such growth forms, quite independently of the specific 

 constitution of the protoplasm, it is very probably 

 concerned in the origin of branches from the axon. As 

 the length of the axon increases, the effect of the tip 

 on the more basal regions must decrease, until these 

 regions are more or less physiologically isolated from it. 

 Under these conditions, and perhaps also in relation to 

 local electrical fields about the axon, budding may 

 occur. Physiological isolation results not only through 

 increase in distance from the dominant region, but also 

 through decrease in activity of this region. In many 

 central neurons growth of the axon ceases a short 

 distance from the cell body and collaterals. In such 

 neurons the appearance of the collaterals is probably 

 associated with the cessation of growth in the main axon. 

 Moreover, conditions such as this within the axon may 

 render it more susceptible to the local electrical fields. 



Kappers has called attention to the fact that the 

 direction of growth of collaterals is usually at first more 

 or less perpendicular to the axon from which they arise 

 and accounts for this fact in terms of his hypothesis. 

 From my point of view this direction of growth may be 

 accounted for as follows: as soon as the growing tip of 

 a collateral is established, its most positive region will 

 be determined farthest away from the axon from which 

 it arises, since this is more or less positive internally, 

 either because growth is still going on, or because of 

 the passage of impulses. Consequently the collateral 

 will tend to grow at right angles to the axon for a certain 



