O TOCHOKD A XD VElt TKHUA L (_ 'OL TJAV. 



451 



FIG. 315. SECTION THROUGH 

 THE VEETEBRAIj COLUMN OF AlIMO- 

 CCETES. (From Gegenbaiir.) 



Ch. notochord ; cs. notochordal 

 sheath ; in. spinal cord ; a. aorta ; 

 r. cardinal veins. 



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 in- 

 terposed (fig. 310, ), 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 to have penetrated 

 through the membrana elastica externa 

 to reach their final situation. The 

 layer with these cells soon increases in 

 thickness, and forms a continuous un- 

 segmented tube of fibrous tissue with 

 flattened concentrically arranged nuclei 

 (fig. 317, VI}. Externally is placed the 

 membraua elastica externa (we/), while 

 within is the cuticular sheath of the 

 notochord. This tube is the cartila- 

 ginous tube spoken of above, and is 

 known as the cartilaginous sheath 

 of the notochord. 



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 Eldsmobranch Fishes I held C A 

 with Gegenbaur that it arose from a layer 

 of cells outside the sheath of the notochord, 

 on the exterior of which the membrana 

 elastica externa was subsequently formed. 

 To this view Gb'tte (No. 419) also gave 

 his adhesion. Schneider has since (No. 

 429) stated that this is not the case, but 

 that, as described above, the membraua 

 elastica externa is formed before the layer 

 of cartilage. I have since worked over 

 this subject again, and am on the whole 

 inclined to adopt Schneider's correction. 



It follows from the above descrip- 

 tion that the cartilaginous tube in 

 question is an essential part of the sheath of the notochord, and 

 that it is to some extent homologous with the notochordal sheath of 

 the Sturgeon and the Lamprey, and not an entirely new formation. 



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

 though part of it subsequently becomes hyaline cartilage. 



292 



s.c 



T/Z e.e 



FIG. 316. LONGITUDINAL SEC- 

 TION THROUGH A SMALL PART OF THE 

 NOTOCHORD AND ADJOINING PARTS OF 

 A SCYLLIUM EMBRYO, AT THE TIME OF 

 THE FIRST FORMATION OF THE CARTI- 

 LAGINOUS SHEATH. 



ch. notochord; sc. sheath of 

 notochord ; n. nuclei of cartilagi- 

 nous shtath ; me.e. membrana elas- 

 tica exterua. 



