SECTION II 

 THE NERVOUS SYSTEM OF VERTEBRATES 



s 



IN these, as in the invertebrata, the central nervous system 

 developed by an involution of the epiblast, revealing thereby its 

 primitive relations to the surface of the body. At an early period 

 in foetal life, shortly after the formation of the two layers of epiblast 

 and hypoblast, a thickening is observed in the epiblast. This 



nc 



me 

 my 



Sit <K,,c <j=a kg.^ !- = * ... -* 



Jw /-' "-^% JO/- > Y;| V 



FIG. 141. Transverse section of human embryocf 2'4 mm. to show developing 

 neural canal. (T. H. BRYCE.) 



nc, neural canal ; me, muscleplate ; my, outer wall of somite ; 



sc, sclerotome. 



thickening soon gives place to a groove, the neural groove (Fig. 141), 

 and the walls of the groove folding over form a canal, the neural canal, 

 which is dilated at the head end of the embryo to form three enlarge- 

 ments known as the cerebral vesicles. 



When first formed the canal is oval in cross-section, its wall being 

 made up of a layer of columnar cells between the outer extremities 

 of which are seen smaller rounded cells. The internal layer of 

 columnar cells send a process peripherally which branches at the end 

 so as to form a close mesh work of fibres. These fibres branch more 

 and more as development progresses, and eventually form the support- 

 ing tissue of the adult central nervous tissue, known as the neuroglia. 

 As the wall of the canal grows in thickness, some of the cells may 

 wander outwards and form neuroglia-cells with numerous radiating 

 branches. In the adult nervous system little is left of these cells 



334 



