PRIMITIVE CEREBRO-SPIXAL CAXAL. 



573 



DEVELOPMENT OF THE CEREBRO-SPINAL AXIS. 



The cerebro-spinal axis is formed from a superficial deposit of blastema, which 

 occupies the whole width of the dorsal furrow, that elongated depression whose 

 margins come together to complete the walls of the cranio-vertebral cavity (p. 15). 

 This layer of blastema increases in 

 thickness in each lateral half, while in Fig. 386. 



A. 



Fig. 386. PRIMITIVE FORM OF THE 

 CEREBRO-SPINAL Axis IN THE EM- 

 BRYO OF THE BIRD. Magnified. 



A and B (from Reichert) outlines of 

 the dorsal aspect of the embryo bird at 

 twenty-four and thirty-six hours of in- 

 cubation. In A, the sides of the pri- 

 mitive groove have united to a great 

 extent and converted it into a canal, 

 dilated at the cephalic extremity, 2 ; 

 6, the cephalic fold of the germinal 

 membrane ; 8, the primordial vertebral 

 masses ; 9, the unclosed lumbar part 

 of the vertebral groove. In B, 10, 11, 

 and 12 indicate the partial division of the 

 cephalic portion of the tube into the 

 three primary vesicles ; 13, the rudi- 

 ment of the eye ; 14, that of the ear. 



C, represents a transverse section of 

 the body of the embryo previous to the 

 closure of the vertebral groove. 1, 

 chorda dorsalis; 2, primitive vertebral 

 groove ; 2 to 3, medullary plates con- 

 tinuous at 3, with 4, the corneous layer 

 of the blastoderm ; 5, the ventral plates 

 of the middle layer ; 6, the lowest or epi- 

 thelial layer; 7, the primordial vertebral 



Fig. 387. 



Fig. 387. TRANSVERSE SECTION OF THE CERVICAL 

 PART OF THE SPINAL CORD OF A HUMAN EMBRYO OF 

 Six WEEKS (from Kolliker). * 



This and the following figure are only sketched, the 

 white matter and a part of the grey not being shaded 

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

 (inferiorly), the part which becomes the anterior com- 

 missure ; at e' (superiorly) the part which becomes 

 the posterior commissure ; 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 g, which points to the grey substance ; p t 

 posterior column ; a, r, anterior roots ; p, r, posterior 

 roots. 



the middle line the primitive groove it remains 

 thin and depressed. The thin middle portion is 



that which forms in the spinal cord the anterior commissure. At the same time 

 that the walls of the cranio-vertebral cavity are completed behind, the lateral margins 

 of the cerebro-spinal axis are also bent backwards and meet together, so as to form 

 a tube ; and this line of junction is the rudiment, in the spinal cord, of the posterior 

 commissure, while the space within the cylinder is the central canal. The closure of 

 the canal first takes place in the cervical region, and subsequently proceeds thence 

 backwards in the dorsal, lumbar, and sacral regions. 



