DEVELOPMENT OF THE SPINAL CORD. 677 



DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM. 

 Spinal Cord. 



Soon after the closure of the medullary or neural canal at its 

 anterior or cranial end, it is dilated in this region into three vesi- 

 cles known as the first, second, and third cerebral vesicles, from 

 which the brain is developed. The spinal cord is formed from 



FIG. 266. 



Transverse section of the spinal column of the human embryo of from 

 nine to ten weeks. (Kolliker.) dm. Dura mater, p'. Columns of Goll. 

 p. Posterior column, pr. Posterior root. na. Arch of vertebra, g. Gan- 

 glion of a spinal nerve, a. Anterior column, ar. Anterior root. ch. Noto- 

 chord. 6. Body of the vertebra, n. Spinal nerve, c. Central canal, e. 

 Epithelium of canal. 



the part of the medullary canal which lies over the chorda dor- 

 salis. The medullary canal is lined by columnar cells derived 

 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- 



