SKELETON. 



137 



the neural process, the other between two of these rings. In 

 development {Arnia) this embolomerous condition is derived 

 from the rhachitomous type by the fusion of hypocentra pleura- 

 lia with the pleurocentra to form one ring (centrum, auct.'), while 

 the other (intercentrura) is developed by a dorsal extension of 

 the hypocentrum arcale. In others it 

 may be that no hypocentra pleuralia occur, 

 the centrum arising by a ventral extension 

 of the pleurocentra. 



In the birds and mammals the vertebra 

 arises at first by what has been called a ver- 

 tebral bow, passing beneath the notochordal 

 sheath and obliquely upwards and back- 

 wards to the posterior limits of the somite, 

 while a little later the centrum proper forms 

 behind the bow. This of course suggests a 

 comparison with the rhachitomous vertebra. 



Concerning the fates of these parts in 

 the higher vertebrates there is a difference 

 of opinion. American students, as a rule, 



regard the pleurocentra as giving rise to the body of the verte- 

 brae in the amniotes, the intercentrum appearing as the chevron 

 bones well known in mammals. In amphibia and teleosts, on 

 the other hand, the vertebral body is said to arise from the 

 intercentrum ; i.e., from the hypocentrum arcale. Thus the ver- 

 tebrae cannot be regarded as exactly homologous throughout the 

 vertebrate phylum. Many European authorities, on the other 

 hand, claim that the centrum of the vertebrates arises from the 

 hypocentrum arcale, and that the pleurocentra either contribute 

 or give rise to the anterior zygapophyses to be mentioned later. 



A third type of vertebra is the phyllospondylous, the rela- 

 tions of which to the foregoing has yet to be made out. In this 

 the vertebral body is composed of right and left halves. This 

 type is found in the fossil Branchiosauridae (stegocephalous 

 batrachia). 



To the parts of the vertebrae so far described others may be 

 added. In the embryo a ligament (interspinous ligament) runs 

 the length of the body just dorsal to the spinal cord. Where 



Fig. 143. Tail verte- 

 brae of extinct stego- 

 cephalous Ewycormits 

 speciosus showing embo- 

 lomerous condition, after 

 Zittell. h, hypocen- 

 trum; ha, hsemal arch; 

 re, neural arch ;/, pleuro- 

 centrum (intercentrum). 



