THE HISTOGENESIS OF THE NERVOUS TISSUES 



313 



The Differentiation of the Neuroblasts into Neurones. — The nerve fibers are 

 developed as outgrowths from the neuroblasts, and a nerve cell with all its pro- 

 cesses constitutes a neurone or cellular unit of the nervous system. The origin 

 of the nerve fibers as processes of the neuroblasts is best seen in the development 

 of the root fibers of the spinal nerves. 



The Efferent or Ventral Root Fibers of the Spinal Nerves. — At the end of 

 the first month clusters of neuroblasts separate themselves from the syncytium 

 in the mantle layer of the neural tube. The outline of the neuroblasts becomes 

 pyriform and from the small end of 

 the cell a slender primary process 

 grows out (Figs. 300 and 301). The 

 process becomes the axis cylinder- of 

 a nerve fiber. The primary processes 

 may course in the marginal layer of 

 the neural tube, or, converging, may 

 penetrate the marginal layer ventro- 

 laterally and form the ventral roots 

 of the 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 early are differen- 

 tiated. These are the neurofibrils 

 and are the conducting elements of 

 the neurones. The cell bodies of the 

 efferent neurones soon become multi- 

 polar by the development of branched 

 secondary processes, the dendrites. 



The Development of the Spinal Ganglia and Afferent Neurones of the 

 Spinal Cord. — The ganglion crest. — After the formation of the neural plate and 

 groove a longitudinal ridge of cells is differentiated on each side where the ecto- 

 derm and neural plate are continuous (Fig. 302 A). This ridge of ectodermal 

 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. 302 C). As development continues they separate into right 

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



NV 



Fig. 301. — Transverse section of the spinal 

 cord from an embryo of the fourth week showing 

 pear-shaped neuroblasts giving rise to ventral root 

 fibers (His in Marshall's Embryology). NC, cen- 

 tral canal of spinal cord; ND, dorsal root of spinal 

 nerve; NI, nuclei of spongioblasts; NV, ventral 

 motor root fibers; NW, ventral funiculi; NZ, 

 neuroblasts. 



