7-18 DEVELOPMENT OF THE NERYOUS SYSTEM. 



grey substance, or the nerve-cells and non-medullated fibres. The 

 cylindrical cells which, from the first, line the whole canal, remain per- 

 manently in the part of it which forms the central canal of the spinal 

 marrow, and frequently present the ciliated structure. 



THE SPINAL MARROW- 



The internal prey substance of the spinal marrow is first formed; 

 the white substance is produced later on the exterior. The sides 

 acquire considerable increased thickness, while the dorsal and ventral 

 parts remain comparatively thin, so that the cavity assumes the appear- 

 ance in section of a slit, which becomes gradually narrower as the 

 lateral thickening increases ; and at last the opposite surfaces uniting 

 in the middle divide the primary central canal into an anterior or lower 

 and posterior or upper part (see figs. 547 and 548). 



The lower of these divisions becomes the permanent central canal, 

 the upper or dorsal is afterwards so far obliterated that it is filled with 

 a septum of connective tissue belonging to the pia mater, and becomes 

 the posterior fissure of the cord (in human anatomy). (Lockliart 

 Clarke, Phil. Trans. 1862.) 



In birds and mammals there is no distinction to be seen at first be- 

 tween the outer or corneous layer of the involuted epiblast and the cells 

 which by their increase more immediately constitute the medullary 

 plates. In batrachia, however, the dark colour of the corneous layer 

 shows it to be distinct from the more strictly nervous layers. In osseous 

 fishes there is no medullary groove or canal at first, but an involution 

 of a solid column of cells, which is subsequently hollowed out for tlie 

 formation of a ventricular cavity. 



The masses of grey matter first formed in the spinal marrow corre- 

 spond chiefly to the anterior columns ; these are succeeded by lateral 

 masses or columns, and somewhat later by small posterior columns. 

 There are at first no commissures except by the passage of the deepest 



Fig. 548. — Transverse Section of the CervicaIi 

 Part of the Spinal Cord of a Human Ejibryoop 

 Six Weeks (from Kolliker). 'f 



Tins and the following figure are only sketched, tJie 

 white matter and a part of the gre.y not being shaded 

 in. r, central canal ; e, its epithelial lining, at e 

 (inferiorly), the i)art which becomes the anterior 

 comniissure ; at c (superiorly), the original place 

 of closure of the canal ; a, the white substance of 

 the anterior columns, beginning to be separated from 

 the grey matter of the interior, and extending round 

 into the lateral column, where it is crossed by the 

 line from [/, which points to the grey substance ; p, 

 posterior column ; ar, anterior roots ; j5r, posterior 

 roots. 



layer of cells across the middle line, but the fibres from the roots of the 

 nerves when formed are traceable into the grey substance of their 

 rcs])cctive anterior and posterior columns. 



The white substance is formed external to or on the surface of the 

 deeper grey substance j but it is not yet determined whether it is 



