452 



SHEATH OF THE NOTOCHORD. 



This sheath forms the basis of the centra of the future vertebrae. 

 In a few adult forms, i.e. Chimsera and the Dipnoi, it retains its primi- 



'P - 



'mel 



a o 



Y. c-tux^. 



Fig. 317. Transveese section through the ventral part of the notochord anp 

 adjoining structures of an advanced scyllium embryo at the root of the tail. 



Vh. cartilaginous sheath of the notochord ; ha. haemal arch ; vp. process to which 

 the rib is articulated ; md. membrana elastica externa ; c)i. notochord ; ao. aorta ; 

 V.cau. caudal vein. 



tive condition, except that in Chima'ra there are present delicate 

 ossified rings more numerous than the arches ; while in the Notidani, 

 Lgemargi and Echinorhini the indications of vertebrae are imperfectly 

 marked out. The further history of this sheath in the forms in which 

 true vertebra? 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. Tlie 

 sheath within becomes unusually thick. 



In the Amphibia and in the Amniota 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 Elasmo- 

 branchii, 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 



