304 



HISTOGENESIS 



spinal nerves. Similarly, the efferent fibers of the cerebral nerves grow out from 

 neuroblasts of the brain wall. Within the cytoplasm of the nerve cells and their 

 primary processes strands of fine fibrils are early differentiated. These, the 

 neurofibrillcR, are the conducting elements of the neurones. The cell bodies of the 

 efferent neurones soon become multipolar by the development of branched 

 secondary processes, the dendrites. 



The Development of the Spinal Ganglia. — After the formation of the neural 

 plate and groove a longitudinal ridge of cells appears on each side where the ecto- 

 derm and neural plate ^ire continuous (Fig. 309 A). This ridge of ectodermal 



Neural cresl 



Ectoderm 



Neural tube 

 Neural crest 



.^T'<e 



Ectoderm 



Ectoderm 

 Neural crest 



S 9 Mesodermal segment 



Fig. 309. — ^Three stages in the development of the ganglion crest in human embryos (after von Len- 



hossek in Cajal) . 



cells is the neural or ganglion crest. When the neural tube is formed and the 

 ectoderm separates from it, the cells of the ganglion crest overlie the neural tube 

 dorso-laterally (Fig. 309 C). As development continues they separate into right 

 and left Unear crests, distinct from the neural tube, and migrate ventro-laterally 

 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. 358). The spinal gangha are arranged segmentally and are 



