NOTOCHORD AND VERTEBRAL COLUMN. 



547 



it the vertebral arches directly rest. In Elasmobranchii the 

 sheath of the notochord undergoes a more complicated series_of 

 changes, which result first of all in the formation of a definite 

 unsegmented cartilaginous tube 1 

 round the notochord, and subse- 

 quently (in most forms) in the 

 formation of true vertebral bodies. 

 Between the membrana elastica 

 externa and the sheath of the 

 notochord a layer of cells becomes 

 interposed (fig. 316, ), which lie 

 in a matrix not sharply separated 

 from the sheath of the notochord. 

 The cells which form this layer 

 appear to be derived from a special 



investment of the notochord, and 



FIG. 315. SECTION THROUGH 



to have penetrated through the THE VERTEBRAL COLUMN OF AMMO- 



c/, 



CCETES. (From Gegenbaur.) 



Ch. notochord ; cs. notochordal 

 sheath; m. spinal cord; a. aorta; 



membrana elastica externa to 



reach their final situation. The 



layer with these cells soon increases 7 '- cardmal vems - 



in thickness, and forms a continuous unsegmented tube of 



fibrous tissue with flattened concentrically arranged nuclei (fig. 



317, Vb}. Externally is placed c f, 



the membrana elastica externa 



(met}, while within is the cuticular 



sheath of the notochord. This 



tube is the cartilaginous tube 



spoken of above and is known as 



the cartilaginous sheath of 



the notochord. 



FIG. 316. LONGITUDINAL SEC- 

 TION THROUGH A SMALL I'ART OF 

 THE NOTOCHORD AND ADJOINING 

 PARTS OF A SCYLLIUM EMBRYO, AT 

 THE TIME OF THE FIRST FORMA- 

 TION OF THE CARTILAGINOUS 

 SIIKATM. 



ch. notochord; sc. sheath of noto- 

 chord; . nuclei of cartilaginous 

 sheath ; nie.e. membrana elastica 

 externa. 



The exact origin of the cartilaginous 

 tube just described is a question of fun- 

 damental importance with reference to 

 the origin of the vertebral column and 

 the homologies of its constituent parts ; 

 but is by no means easy to settle. In the account of the subject in my 

 memoir on Elasmobranch Fishes I held with Gegenbaur that it arose from 



1 This tube consists of a peculiar form of fibrous tissue rather than true cartilage, 

 though part of it subsequently becomes hyaline cartilage. 



352 



