138 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



this passes between the dorsal ends of the neural processes it 

 becomes converted into cartilage, thus giving rise to an addi- 

 tional element (spinous process or neural spine), which, together 



FIG. 144. Fifth to seventh caudal vertebrae of Perameles gunni. h, haemal 

 arch ; n, neural processes ; /, transverse processes. 



with the two neural processes, form a neural arch enclosing and 

 protecting the spinal cord. In the caudal region of the ichthy- 

 opsida and some higher forms, the vertebra is completed below by 

 a similar haemal arch, which encloses the caudal artery and vein. 



This arch is composed of a pair of hae- 

 mal processes (haemapophyses) and a 

 haemal spine. These various parts of 

 the vertebrae arise separately ; but they 

 exhibit in recent forms a tendency to 

 fuse together in the adult, the fusion 

 being most complete in the higher 

 groups. 



The vertebrae are laid down at an 

 early stage in development, and their 

 number is not subsequently increased. 

 Increase in length of body is therefore 

 Caudal verte- accomplished by longitudinal growth of 

 the centra of the vertebrae. In the 

 fishes additions are made, as it were, in 

 layers, on the circumference of the cen- 

 trum first formed, each new layer being 

 slightly longer than its predecessor. As 



a result the centrum becomes concave on either end, is am- 

 phiccelous. The parts of the centrum first formed prevent 

 any farther increase of the notochord in the intravertebral 

 regions ; but intervertebrally it expands, filling up the cavities 

 between the successive vertebrae, and thus assuming the appear- 



FIG. 145 



bra of alligator, az, prezyga- 

 pophysis; c, centrum; d t 

 diapophysis ; h, haemal arch; 

 ns, neural spine ; /z, postzy- 

 gapophysis. 



