302 



HISTOGENESIS 



extending peripherally; (2) a middle mantle or nuclear zone, and (3) an outer or 

 marginal zone, non-cellular, into which nerve fibers grow. The ependymal zone 

 contributes cells for the development of the mantle layer {D). The cellular 

 mantle layer forms the gray substance of the central nervous system, while the 

 fibrous marginal layer constitutes the white substance if the spinal cord. 



The primitive germinal cells of the neural tube divide by mitosis and give 

 rise to the ependymal cells of the ependymal zone and to indifferent cells of the 

 mantle layer. From these latter arise spongioblasts and neuroblasts (Fig. 306). 

 The spongioblasts are transformed into neuroglia cells and fibers, which form the 

 supporting tissue of the central nervous system; the neuroblasts are primitive 

 nerve cells, which, by developing cell processes, are converted into neurones. The 

 neurones are the structural units of the nervous tissue. 



Ependymal cells 



Germinal cells 

 Indiferent cells 



Mitotic indiferent cells 



Neuroblasts 



Neuroglia cells 



P Q F '-' 



Fig. 306.— Diagrams showing the differentiation of the cells in the wall of the neural tube and the theo- 

 retical derivation of the ependymal cells, neuroglia cells and neuroblasts (after Schaper). 



The Diflferentiation 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 neuroblasts become pear-shaped 

 and from the small end of the cell a slender primary process grows out (Figs. 307 

 and 308). 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 



