196 THE ORIGIN OF THE NERVOUS SYSTEM 



the more general gradients, because at the time of its 

 development the simpler general gradients of earlier 

 stages have undergone complication in various ways. 



THE NEURONS OF THE SPINAL GANGLIA 



The neurons of the spinal ganglia of the vertebrates 

 present certain special problems. When the neural 

 tube arises by the development of the neural folds and 

 the borders of the neural plate (Fig. 62) and the meeting 

 and fusion of these folds (Fig. 63), some of the cells of 

 the dorsal border of the folds, the neural crest, are not 

 incorporated in the neural tube, but migrate ventrally 

 on each side of it (Fig. 64), forming, according to 

 Johnston (1906, p. 50), segmental series of flaps attached 

 to the dorsolateral surface of the cord and hanging 

 down on its lateral surface. Later these flaps may 

 separate entirely from the cord and form an aggregation 

 of cells which constitutes the primordium of the spinal 

 ganglion and lies between the cord and the mesoderm 

 (Fig. 65). 



In the process of differentiation each of these cells 

 which becomes a neuron gives rise first to an outgrowth 

 which extends dorsomedially and enters the cord, and 

 somewhat later to a second outgrowth, which grows in 

 the opposite direction (Fig. 60). Both of these out- 

 growths are structurally similar to the axons of other 

 neurons and both may develop medullation. Function- 

 ally they constitute the afferent or sensory paths, and 

 the direction of functional conduction in the peripheral 

 outgrowth is toward the cell body instead of away from 

 it, as in other axons. The question which of these out- 

 growths is axon and which dendrite, or whether both 



