DEVELOPMENT OF THE SPINAL CORD. 



675 



from the epiblast, which, shortly after they are shut off from the 

 general epiblast, develop at the sides of the canal, so as to narrow 

 the lumen of the tube by the increase in thickness of its sides. 

 The upper and lower parts of the canal do not, however, become 

 thickened. The lateral walls approximate to the centre, decreas- 

 ing laterally the lumen of the canal, which becomes narrow in 

 the middle with a dilatation above and below. The lateral walls 



FIG. 266. 



TransTerse section of the spinal column of the human embryo of from nine to ten 

 weeks. (K&lliker.) dm. Dura mater, p'. Columns of Goll. p. Posterior column, pr. 

 Posterior root. no. Arch of vertebra, g. Ganglion of a spinal nerve, o. Anterior 

 column, ar. Anterior root. ch. Notochord. b. Body of the vertebra, n. Spinal nerve, 

 c. Central canal, e. Epithelium of canal. 



of the canal, thus approximated, unite in their centre, and con- 

 vert the medullary canal into two separate tubes, a dorsal and a 

 ventral. 



The lower or ventral tube of the divided canal becomes the 

 central canal of the spinal cord, and the columnar cells of the 

 epiblast form a lining of ciliated columnar epithelium. 



The epiblast at the lower part of the canal becomes converted 



