VERTEBRAL COLUMN, RIBS, AND STERNUM 303 



Dipnoi. In all other forms the vertebrae arise outside the 

 notochord and do not invade its sheath. These are called 

 perichordal vertebrae. 



The vertebrae of the higher bony fish are compact bony 

 structures obliterating the notochord. Amia is interesting 

 with regard to its vertebral column, for in the region of the 

 tail there are what look like vertebrae with neural and haemal 

 arches alternating with vertebrae without. Those vertebrae 

 with the neural and haemal arches are the basidorsals and basi- 



FiG. 152. — Transverse sections through the developing vertebral column of 



Scy Ilium embryos. 



A, early stage ; B, late stage (in the region of the tail), b d, basidorsal 

 bv, basiventral ; ca, caudal artery ; cv y caudal vein ; ee, elastica externa ; 

 ha, haemal arch ; hs, haemal spine ; n, notochord ; na, neural arch ; nc, 

 nerve-cord ; ns, neural spine. 



ventrals ; those without are the interdorsals and interventrals. 

 The neural arches are always formed from basidorsals and the 

 haemal arches from basiventrals. 



The vertebrae are formed from the sclerotome, which is 

 segmented. The anterior part of the sclerotome in each seg- 

 ment gives rise to the interdorsals and interventrals, while 

 the posterior part produces the basidorsals and basiventrals. 

 Later it is found that the basidorsals and basiventrals of one 

 segment fuse on to the interdorsals and interventrals of the 



