NO TOO HO RD AND VERTEBRAL COLUMN. 453 



view of its extreme delicacy and unimportant function in Elasmo- 

 branchii is not difficult to do) and this cellular sheath would then 

 obviously be homologous with the cartilaginous tube in question. In 

 the Amniota an external sheath of the notochord cannot be traced 

 as a distinct structure, but the connective tissue surrounding the 

 notochord and spinal cord is simply differentiated into the vertebral 

 bodies and vertebral arches. 



Vertebral arches and Vertebral bodies. 



Cydostomata. The Cyclostomata are the most primitive forms 

 in which true vertebral arches are present. Their ontogeny in this 

 group has not been satisfactorily worked out. It is however notice- 

 able in connection with them that they form for the most part isolated 

 pieces of cartilage, the segmental arrangement of which is only im- 

 perfect. 



Elasmobranchii. In the Elasmobranchii the cells forming the 

 vertebral arches are derived from the splanchnic layer of the meso- 

 blastic somites. They have at first the same segmentation as the 

 somites (fig. 313, Fr), but this segmentation is soon lost, and there is 

 formed round the notochord a continuous sheath of embryonic con- 

 nective tissue cells, which gives rise to the arches of the vertebras, 

 the tissue forming the dura mater, the perichondrium, and the general 



investing connective tissue. 



The changes which next follow result in what has been known 

 since Remak as the secondary segmentation of the vertebral column. 

 This segmentation, which occurs in all Vertebrata with true verte- 

 brae, is essentially the segmentation of the continuous investment 

 of the notochord and spinal cord into vertebral bodies and vertebral 

 arches. It does not however follow the lines of the segmentation 

 of the muscle-plates, but is so effected that the centres of the verte- 

 bral bodies are opposite the septa between the muscle-plates. 



The explanation of this character in the segmentation is not difficult to 

 find. The primary segmentation of the body is that of the muscle-plates, 

 which were present in the primitive forms in which vertebras had not 

 appeared. As soon however as the uotochordal sheath was required to be 

 strong as well as flexible, it necessarily became divided into a series of 



segments. 



The condition under which the lateral muscles can best cause the 

 flexure of the vertebral column is clearly that each myotome shall be 

 capable of acting on two vertebra? ; and this condition can only be fulfilled 

 when the myotomes are opposite the intervals between the vertebrae. For 

 this reason, when the vertebras became formed, their centres were opposite 

 not the middle of the myotomes but the inter-muscular septa. 



These considerations fully explain the characters of the secondary 

 segmentation of the vertebral column. On the other hand the primary 

 segmentation (fig. 313) of the vertebral rudiments is clearly a remnant 

 of a condition when no vertebral bodies were present ; and has no greater 

 morphological significance than the fact that the cells of the vertebras 



