NOTOCHORD AND VERTEBRAL COLUMN. 549 



indications of vertebrae are imperfectly marked out. The further 

 history of this sheath in the forms in which true vertebrae are 

 formed can only be dealt with in connection with the formation 

 of the vertebral arches. 



In Teleostei there is present, as in Elasmobranchii, an elastica externa, 

 and an inner notochordal sheath. The elastica externa contains, according 

 to Gotte, cells. These cells, if present, are however very difficult to make 

 out, but in any case the so-called elastica externa appears to correspond with 

 the cartilaginous sheath of Elasmobranchii together with its enveloping 

 elastica, since ossification, when it sets in, occurs in this layer. The sheath 

 within becomes unusually thick. 



In the Amphibia and in the Amn'iota no membrane is 

 present which can be identified with the membrana elastica 

 externa of the Elasmobranchii, Teleostei, etc. In Amphibia 

 (Gotte) there is formed round the notochord a cellular sheath, 

 which has very much the relations of the cartilaginous tube 

 around the notochord of Elasmobranchii, and is developed in 

 the same way from the perichordal connective tissue cells. It 

 is only necessary to suppose that the membrana elastica externa 

 has ceased to be developed (which in view of its extreme delicacy 

 and unimportant function in Elasmobranchii 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. 



Cyclostomata. 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 noticeable in connection with them that they form for 

 the most part isolated pieces of cartilage, the segmental 

 arrangement of which is only imperfect. 



Elasmobranchii. In the Elasmobranchii the cells forming 

 the vertebral arches are derived from the splanchnic layer of the 

 mesoblastic somites. They have at first the same segmentation 



