1 88 THE ORIGIN OF THE NERVOUS SYSTEM 



development of the spinal cord. This is probably, like 

 the axon outgrowth, a result of the electrical polariza- 

 tion of the cell, the side toward the cathode being more 

 positive, more active physiologically, and growing more 

 rapidly than, or even at the expense of, the other side. 

 Apparent migrations resulting from such differences in 

 relation to an axial gradient are of frequent occurrence 

 in organisms. One of the most striking examples is 

 found in the stolons of various hydroids. In the 

 absence of food the tip of the stolon, which represents 

 the high end of the axial gradient, grows at the expense 

 of lower levels, and the stolon apparently creeps forward 

 and may even become separated from the parent organ- 

 ism through the resorption of its more basal regions as 

 nutrition for the growing tip. Such free stolons appar- 

 ently wander about over the surface to which they are 

 attached, always growing at the tip and undergoing 

 resorption at the base, until they finally use up their 

 own substance and disappear, or else, under certain con- 

 ditions, give rise to a new hydranth and begin to feed. 

 The apparent migrations of the neuroblasts and later 

 of the neurons (p. 200) are perhaps due to such growth 

 on the electropositive and resorption on the electro- 

 negative side of the cell. 



The direction of growth of the axons is not neces- 

 sarily altered by their emergence from the neural tube. 

 Since they are protoplasmically continuous with the cell 

 bodies in the tube, the electrical polarization of their 

 tips may still remain the same. Consequently the 

 general direction of growth of these axons will be down 

 a resultant of the chief physiological gradients of the 

 developing body*. 



