DEVELOPMENT OF THE NERVOUS SYSTEM 



733 



lame of the nucleus or the location of the cell body from which the axon or fiber 

 irises preceding that of the nucleus or location of its termination. A given topo- 

 graphical area seldom represents a pure tract, as in most cases fibers of different 

 systems are mixed. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



The entire nervous system is of ectodermal origin, and its first rudiment is seen 

 in the neural groove which extends along the dorsal aspect of the embryo (Fig. 

 17). By the elevation and ultimate fusion of the neural folds, the groove is con- 

 verted into the neural tube (Fig. 19). The anterior end of the neural tube becomes 

 expanded to form the three primary brain-vesicles; the cavity of the tube is sub- 

 sequently modified to form the ventricular cavities of the brain, and the central 

 canal of the medulla spinalis; from the w r all the nervous elements and the neuroglia 

 of the brain and medulla spinalis are developed. 



Roof-plate 



Oval bundle 

 Posterior nerve root 



Central canal 

 Ependymal layer 



Mantle layer 



Anterior nerve roottt 



Marginal layer 

 Floor-plate 

 FIG. 640. Section of medulla spinalis of a four weeks' embryo. (His.) 



The Medulla Spinalis. At first the wall of the neural tube is composed of a 

 single layer of columnar ectodermal cells. Soon the side-walls become thickened, 

 while the dorsal and ventral parts remain thin, and are named the roof- and floor- 

 plates (Figs. 640, 642, 643). A transverse section of the tube at this stage presents 

 an oval outline, while its lumen has the appearance of a slit. The cells which 

 constitute the wall of the tube proliferate rapidly, lose their cell-boundaries and 

 form a syricytium. This syncytium consists at first of dense protoplasm with 

 closely packed nuclei, but later it opens out and forms a looser meshwork with 

 the cellular strands arranged in a radiating manner from the central canal. Three 

 layers may now be defined an internal or ependymal, an intermediate or mantle, 

 and an external or marginal. The ependymal layer is ultimately converted into the 

 ependyma of the central canal; the processes of its cells pass outward toward 

 the periphery of the medulla spinalis. The marginal layer is devoid of nuclei, and 

 later forms the supporting framework for the white funiculi of the medulla spinalis. 

 The mantle layer represents the whole of the future gray columns of the medulla 

 spinalis; in it the cells are differentiated into two sets, viz., (a) spongioblasts or 

 young neuroglia cells, and (b) germinal cells, which are the parents of the neuroblasts 





