VERTEBRAL COLUMN. 



&d 



FlO 



a Inugitudiua 



mented ; in other words it becomes differentiated into alter- 

 nately short fibrous and longer cartilaginous portions (Fig. 34). 

 The fibrous portions become the intervertebral ligaments, the 

 cartilaginous , portions the 

 bodies or centra of the verte- 

 brae. The sheath thickens in 

 the central part of the verte- 

 bral regions and constricts the 

 notochord, so that the noto- 

 chord assumes a beaded form, 

 being narrowest in the middle 

 of the vertebral regions and 

 widest in the intervertebral 

 (Fig. 34). In this way a bi- 

 concave or amphicoelous cen- 

 trum — a form eminently 

 characteristic of fishes — arises. 

 When the centra are formed 

 entirely or mainly from the 

 chordal sheath they are called 

 cJiordo-centrous {Dipnoi, Elas- 

 mobranchii). But it frequently 

 happens that they are rein- 

 forced by cartilage derived 

 from the arch-tissue. The arch-tissue arises from the meso- 

 blastic tissue (skeletogenous layer), which surrounds the noto- 

 chord, and is continued dorsally round the spinal cord. Four 



special concentrations of this tissue are 

 formed adjoining the notochord, two dor- 

 sal and two ventral. In these the 

 chondrifications which give rise to the 

 neural and haemal arches begin. The 

 neural arches do not always completely 

 enclose the spinal canal, but are supple- 

 mented by the intercalated pieces (Fig. 

 terior' trvmk-verte- 35) which are placed between them, i.e. 

 (after" Hasse"^ from intervertebrally. The haemal arches may 

 areh°w^ithVoramen"for ^Iso be Supplemented by intercalated 



anterior root; in in- „,'^„„„ rni i-i • i 



tercaiated piece witii picces. Ihcse cartilagmous arches may 

 roSTAhaemaT^ch.' Spread out round the notochord outside 



34. — a Diagram of 

 section of the vertebral column of a 

 Teleostean with vertebral constriction of 

 the notochord (from Claus). b vertebrae 

 of a bony fish, ch notochord ; D neural 

 spine ; i)' haemal spine ; J interverte- 

 bral ligament ; k body of vertebra ; 06 

 neural arch ; iJ rib ; Wk vertebral body. 



