HISTOGENESIS OF THE NERVOUS TISSUES 



301 



sharp outlines and form a compact, cellular syncytium which is bounded, 

 on its outer and inner surfaces, by an external and internal limiting mem- 

 brane (B}. In a 10 mm. embryo the cellular strands of the syncytium are 

 arranged radially and nearly parallel (D) . The nuclei are now so grouped 

 that there may be distinguished three layers: (i) an inner ependymal zone, 

 with cells abutting on the internal limiting membrane, their processes 

 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 of 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 



Ependymal cells 



Neuroblasls 



Neuroglia cells 



FIG. 306. Diagrams showing the differentiation of the cells of the neural tube (after Schaper). 



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 neurons. The neurons are the structural 

 units of the nervous tissue. 



The Differentiation of Neuroblasts into Neurons. The nerve fibers 

 are developed as outgrowths from the neuroblasts, and a nerve cell with 

 all its processes constitutes a neuron 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. 



This neuron concept of the development of the nerve fibers is the one generally accepted 

 at the present time. It assumes that all axons and dendrites are formed as outgrowths 

 from nerve cells, an hypothesis first promulgated by His. The embryological evidence is 



