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 
