DEVELOPMENT OF THE SKELETON. 



729 



the posterior, as matrix or forerunner of the bone and other structures 

 which belong to the vertebral column. Each nerve then comes thus to 

 be placed in front of the future permanent vertebra with the proto- 



Fig. 529. 



V ~ 



Fig. 529. — Transverse section through the Dorsal region of an Embryo-Chick, 

 END of Third Day (from Foster and Balfour). 



Am, amnion ; mp, muscle plate ; c v, cardinal vein ; Ao, dorsal aorta at the point 

 where its two roots begin to join ; Ch, notochord ; Wd, Wolffian duct ; Wh, com- 

 mencement of formation of Wolffian body ; tip, epiblast ; so, somatopleure ; liy, hypo- 

 blast. The section passes through the place where the alimentary canal Qiy) com- 

 municates with the yolk-sac. 



vertebral division of which it was originally connected. In the inner 

 or central part of the primordial vertebrae a different kind of change 

 occurs, first, by the amalgamation or fusion of the protovertebral 

 masses, and subsequently by their subdivision in such a manner that the 

 intervertebral disc arises on a level with or opposite the centre of each 

 protovertebral mass, and the blastema, out of which a permanent verte- 

 bral body is formed, is made up by the union of two parts, an anterior 

 and a posterior, the first of these being derived from the hinder part 

 of the preceding protovertebral mass of the same number, the other part 

 being supplied by the anterior section of the protovertebral mass 

 immediately following. 



This is a somewhat complicated change ; and the more difficult to be followed 

 that it would appear that the original division between the protovertebral masses 

 disappears previous to a new segmentation taking place. Thus it results that, 

 as respecta the centrum or body part, the posterior half of one protovertebra is 



