502 



THE NEKVOUS SYSTEM. 



Such fibres are constituent elements of the facial, glosso-pharyngeal, and in some 

 animals also the vagus cerebral nerves (Fig. 443), in connexion with the ganglia 

 of which these epibranchial placodes are formed (Froriep and Streeter). The 

 observations of Professor J. P. Hill upon embryos of Echidna seem to suggest that 

 in mammals these gustatory neuroblasts are derived from the entoderm. 



When first formed, the neural tube is compressed from side to side and presents 

 an elliptical outline in transverse section (Fig. 444). The two side walls 

 are very thick, whilst the narrow dorsal and ventral portions of the wall are thin, 

 and are termed the roof-plate and floor-plate respectively (Fig. 444). The cavity 

 of the tube in transverse section appears as a narrow slit. The wall of the neural 

 tube consists at first of low columnar epithelium arranged in a fairly regular 

 series, but with a certain number of large spherical so-called germinal cells 

 scattered between the columns. But this regular disposition as a single layer 



Funiculus posteno 



Sensory 

 ganglion 



Marqinal 

 \ayer-- 



Commissural fibre 



-Anterior nerve root 



FIG. 444. DIAGRAM OF TRANSVERSE SECTION OF EARLY NEURAL TUBE. 



of cells does not last long. For even by the second week the rapid proliferation 

 of the cells has led to a marked increase in the thickness of the side wall and 

 a scattering of the more numerous nuclei, apparently irregularly, throughout its' 

 substance (Fig. 444). The latter consists of a network of protoplasm in whicl 

 definite outlines of cells cannot be detected. As growth proceeds the innermosl 

 part of this nucleated protoplasmic syncytium becomes condensed to form e 

 delicate membrane termed the internal limiting membrane, which lines th 

 lumen of the tube, whilst its outermost part presents a similar relation to ai 

 external limiting membrane, which invests the outer surface of the tube. To 

 ward the end of the first month the side walls of the tube show signs of ; 

 differentiation into three layers. Next to the central canal there is an epithelial 

 like arrangement of the innermost cells of the syncytium, forming the ependyms 

 Then there is an intermediate layer crowded with nuclei, hence known as th 

 nuclear or mantle layer. On the surface is a layer singularly free from nucle 

 which is called the non-nuclear or marginal layer. The germinal cells ar 



