NEBVOUS SYSTEM 



95 



spoken of as the germinal zone. The second stage is characterised by a disap- 

 pearance of cell-outlines, so that the wall of the canal appears formed of continuous 

 protoplasmic columns. These then break up (third stage) into a mesh-work 



/mlial <v;/ ///////.< 



<^f protopltum yrniinal zonr 



' 



.r'~r 4 





s clivid ing nuclei 



i/i/. mecZ. lam. 



FIG. 130. SECTION OF THE WALL OF THE NEURAL CANAL FROM AN EMBRYO PIG, 



JUST AFTER THE CLOSURE OF THE NEURAL GROOVE. (Hardesty). 



(terminal zone 



5^ ^7>. ?^7):^^">'7S x^J-r^ """ / ^ r 



^^5fe^ >^/^r;E) :S N ^?y ; 



. med. lam. 



I 



jria 



^-^^ 



(I int. med. lam. 



\ v ' 



-V 



reticular zone 



)>ucl>>(ir zone 



germinal zone 



FIG. 131. SECTIONS OF THE WALL OF THE NEURAL CANAL OF THE PIG AT LATER STAGES THAN 

 THAT SHOWN IN FIG. 130. (Hardesty). 



A, from an embryo of 7 mm. ; B, from an embryo of 10 mm. ; g, dividing 

 nuclei in germinal zone ; r, filaments of myelospongium. 



