Soon after the closure of the medullary or neural canal at its anterior or cranial end, it is dilated in this region into three vesicles, known as the first, second, and third cerebral vesicles, from which the brain is developed. The spinal cord is formed from that part of the medullary canal which lies over the chorda dorsalis. 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. The upper and lower parts of the canal do not become thickened.
The side walls approximate to the centre, decreasing laterally the lumen of the canal, which becomes narrow in the middle with a dilatation above and below. The lateral walls of the canal, thus approximated, unite in the centre, and convert 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 into the anterior gray columns, in connection with which arise the anterior roots of the spinal nerves; while at the upper part the posterior gray columns are formed in connection with the posterior roots of the spinal nerves and their ganglia. The white columns are thought by some authors to be derived from the mesoblast surrounding the canal, but by others they are assigned to the epiblast.
The upper or dorsal canal becomes converted into a fissure by the absorption of its roof, and is thus changed into the posterior fissure of the spinal cord.
Fig. 284. 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 vetrebra.
g. Ganglion of a spinal nerve.
a. Anterior column. ar Anterior root. ch. Notochord.
b. Body of the vertebra. n. Spinal nerve.
c. Central canal.
e. Epithelium of canal.
The anterior fissure is formed by the down-growth of the anterior columns, which diverge, leaving between them an interval which becomes occupied by the pia mater.
The commissures are not formed between the lateral halves of the cord until later. The gray commissure appears first.
Fig. 285. Transverse section of the spinal cord of a chick of seven days. (Foster and Balfour).
ep. Epithelium lining the medullary canal, pf. Part of the cavity of the medullary canal which becomes the posterior fissure, spc. Permanent medullary tube or central canal of the spinal cord. agc. Anterior gray commissure, af. Anterior fissure, not yet well formed, c. Tissue filling in the upper part of the posterior fissure, pc. Cells forming the posterior gray matter, pew. Posterior while column, ct. Mesoblast surrounding the spinal cord. lew. Lateral white column, acw. Anterior white column, ac. Cells forming the anterior gray matter.