676 MANUAL OF PHYSIOLOGY. 



into external parts, the muscle plates, which form eventually the 

 dorsal and other muscles, and internal parts which become the 

 permanent vertebrae. 



From these inner portions processes of mesoblast grow upwards 

 over the medullary canal to meet with processes from the pro- 

 tovertebrae of the opposite side. Mesoblastic tissue also grows 

 inwards between the medullary canal and the notochord, and 

 between the notochord and the subjacent hypoblast. 



These projections beneath the notochord meet with projections 

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

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

 cell mass. 



The portions of the proto vertebrae 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 permanent 

 vertebra. 



The vertebrae do not then correspond to the original proto ver- 

 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 iutervertebral disk. 



In connection with the vertebrae in the dorsal region, processes 

 grow horizontally, these are the rudiments of the ribs. 



