VERTEBR.^ 



21 



accordingly as they lie above or below the notochordal axis. 

 Correspondingly caudineurals and caudihaemals arise from the 

 posterior half sclerotome. Hence there are two pairs of neurals and 

 two of haemals — eight elements in all — to each original pair of 

 myotomes. 



The order of parts concerned in the development of a vertebra is: i, inter- 

 segmental blood vessel (fig. 19); 2, ventral nerve root; 3, cranineural and crani- 

 haemal; 4, dorsal nerve root; 5, caudineural and 

 caudihaemal; 6, intersegmental vessel. The interseg- 

 mental vessels lie in the plane of the myosepta and 

 alternate with the myotomes, thus affording a criterion 

 of metameric limits. 



The vertebrae arise from these various parts, 

 but there is uncertainty in some cases as to 

 the role played by each. There is one great 

 difference between Elasmobranchs and higher relations of haif-scieroto- 



mes and their products 

 to nerves and blood 

 vessels in Elasmobranchs 

 (based on Schauinsland). 

 c, vertebral centra; cd, 

 cr, caudal and cranial 

 parts; d, v, dorsal and 

 ventral nerve roots; i, 

 intercentra; b, interseg- 

 mental blood vessel. 



Vertebrates; in the former cells from the 



sclerotomic elements just mentioned break 



through the elastica externa (fig. 16), invade 



the notochordal sheath, and may chondrify 



there. By extending in all directions these 



invading cells may meet above and below, 



forming cartilage rings around the notochord. 



In those higher Vertebrates where the history is known, cartilage 



cells do not invade the sheath, but surround the elastica, forming 



rings in that position. The sheath is greatly reduced in Amniotes 



and the two elasticae are not differentiated. 



In the tail of Cyclostomes (fig. 234) the neural elements extend up on the 

 sides of the spinal cord, forming neurapophyses, their bases resting on the noto- 

 chordal sheath, while haemapophyses are formed in the same way on either side 

 of the caudal blood vessels. There are no cartilages formed in the trunk of 

 Myxinoids, but Petromyzon (fig. 83) has neurapophyses there. Since there are 

 two half-sclerotomes to a myotome, there are two incomplete arches (no spinous 

 process) to each original somite. 



In Gnathostomes cranial and caudal elements develop unequally, 

 the caudals being the larger and forming the greater part of the whole 

 of the arches, the cranials being less developed. In lower Verte- 

 brates (especially in some extinct Ganoids and Stegocephals) parts 

 resulting from the chondrification of neurals and haemals remain 



