HISTOGENESIS 



to a position between the neural tube and myotomes. In this position the 

 ganglion crest forms a band of cells extending the whole length of the spinal cord 

 and as far cranially as the otic vesicles. At regular intervals in its course along 

 the spinal cord the proliferating cells of the crest give rise to enlargements, the 

 spinal ganglia (Fig. 340) . The spinal ganglia are segmentally arranged and con- 

 nected at first by bridges of cells which later disappear. In the hind-brain region 

 certain ganglia of the cerebral nerves develop from the crest but are not seg- 

 mentally arranged. 



FIG. 302. Three stages in the development of the ganglion crest in human embryos (after von 

 Lenhossek in Cajal). a, ectoderm; b, neural tube; c, mesodermal segment; G, ganglioblasts. 



The Differentiation of the Afferent Neurones. The cells of the spinal ganglia 

 differentiate into (i) ganglion cells and (2) supporting cells, groups which are 

 comparable to the neuroblasts and spongioblasts of the neural tube. The neuro- 

 blasts of the ganglia become fusiform, develop a primary process at either pole 

 and thus these neurones are of the bipolar type. The centrally directed processes 

 of the ganglion cells converge and by elongation form the dorsal roots. They 

 penetrate the dorso-lateral wall of the neural tube, bifurcate and course cranially 

 and caudally in the marginal layer of the spinal cord (Fig. 300, d). By means of 

 branched processes they anastomose with the neurones of the mantle layer. The 

 peripheral processes of the ganglion cells as the dorsal spinal roots join the ventral 



