VERTEBRAL COLUMN. 



59 



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 

 chordo-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. 



FIG. 35. Three pos- ~\ i i i -, 



terior trunk-verte- <* D ) which are placed between them, i.e. 



brae of CentTophorus , , -i -,-, -m , 



(after Hasse from mtervertebrally. The haemal arches may 



Gegenbaur). n neural i r i -, -, 



arch with foramen for also be supplemented by intercalated 



anterior root ; in in- mi , .1 



tercaiated piece with pieces. These cartilaginous arches may 

 haemafarch 1 * spread out round the notochord outside 



FIG. 34. a Diagram of a longitudinal 

 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 ; D l haemal spine ; J interverte- 

 bral ligament ; k body of vertebra ; 06 

 neural arch : R rib : Wk vertebral body. 



