674 MANUAL OF PHYSIOLOGY. 



These projections beneath the notochord meet with projections 

 from a mass of the mesoblast, which lies between the protover- 

 tebrae and the cleft mesoblast, which is known as the intermediate 

 cell mass. 



The portions of the protovertebrae above the medullary canal 

 form the arches of the vertebrae; from those surrounding the 

 notochord the bodies of the vertebrae are developed. 



The outer part of each protovertebra divides into an anterior 

 or pre-axial part, from which arises the ganglion of a spinal nerve, 

 and into a posterior or post-axial part. 



After this the original lines of separations between the proto- 

 vertebrae disappear, and the spinal column is fused into a carti- 

 laginous mass. New segmentation now appears in the centre of 

 each original protovertebra, midway between the primary divi- 

 sions. Thus the vertebral column is divided into a number of 

 component parts, each of which is destined to become a perma- 

 nent vertebra. 



The vertebras do not then correspond to the original protover- 

 tebrae, but rather to the posterior half of that which lay in front 

 of the primary division joined to the anterior half of the one be- 

 hind. The ganglia of the spinal nerves, therefore, by this arrange- 

 ment, instead of belonging to the front of the vertebra, become 

 joined to the posterior part of the vertebra, to which they belong. 



The notochord atrophies with ossification of the vertebrae, and 

 finally is represented only by a mass of soft cells in the centre of 

 an intervertebral disk. 



In connection with the vertebrae in the dorsal region processes 

 grow horizontally ; these are the rudiments of the ribs. 



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 

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

 salis. The medullary canal is lined by columnar cells derived 



