DEVELOPMENT OF THE SPINAL CORD. 



59 



spongioblasts. This may be taken to represent the white matter of the cord at this 

 stage (all the rest representing grey substance) : but there are at first no nerve fibres 

 in it, the only structures which can be at all compared to nerve fibres being the 

 prolongations of the neuroblasts, and these lie either as arcuate fibres altogether in 

 the outer part of the grey substance, or are passing out of the cord as the beginnings 

 of the anterior roots from a mass of neuroblasts which forms the rudiment of the 

 anterior cornu of grey matter (fig. 65). This mass constitutes in the human embryo 

 of six weeks (fig. 66) the chief portion of each half of the cord. It forms a con- 

 siderable projection which laterally almost reaches the surface, but ventrally is sepa- 

 rated from it by a thickening of the external or radial zone, due to the appearance of 

 longitudinally coursing nerve fibres within it : this is the beginning of the anterior 



Fig. 65. SECTION OF SPINAL CORD OF FOUR WEEKS HUMAN EMBRYO (His). 



The posterior roots are continued within the cord into a small longitudinal bundle which is the 

 rudiment of the posterior white column. The anterior roots are formed by the convergence of the 

 processes of the neuroblasts. The latter, along with the elongated cells of the myelospongium compose 

 the grey matter. The external layer of the cord is traversed by radiating fibres which are the outer ends 

 of the spongioblasts. The anterior commissure is beginning to appear. This figure is much more 

 magnified than the next one. 



Fig. 66. TRANSVERSE SECTION OF THE CERVICAL PART OF THE SPINAL CORD OF A HUMAN 



EMBRYO OF six WEEKS (from Kblliker). ^ 



e, central canal ; e, its epithelial lining ; at e' (superiorly), the original place of closure of the canal ; 

 a, the white substance of the anterior columns ; g, grey substance of antero-lateral horn ; p, posterior 

 column ; ar, anterior roots ; pr, posterior roots. 



white column (a). By this time, also, although to a rather less extent, the posterior 

 white columns have, simultaneously with the posterior roots, begun to make their 

 appearance on either side of the narrow dorsal part of the neural canal (p). There is, 

 however, only a relatively thin layer of grey matter (neuroblasts) separating the 

 posterior white columns from the palisade-like lining of the canal, and as yet no 

 sign of nerve fibres in the situation of the lateral columns, which are only repre- 

 sented by a thin layer of the radial myelospongium. The roof and floor of the 

 canal are also quite thin and undeveloped. 



At this period there is still no sign of either anterior or posterior (dorsal or 

 ventral) fissures of the cord. These become formed as the cornua of the grey matter 

 grow out from the central mass, and as the anterior and posterior white columns 



