548 



SHEATH OF THE NOTOCHORD. 



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 

 Gotte (No. 419) also gave his adhesion. Schneider has since (No. 429) 

 stated that this is not the case, but that, as described above, the membrana 

 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 description that the cartilaginous 

 tube in question is an essential part of the sheath of the noto- 

 chord, and that it is to some extent homologous with the noto- 

 chordal sheath of the Sturgeon and the Lamprey, and not an 

 entirely new formation. 



This sheath forms the basis of the centra of the future 

 vertebrae. In a few adult forms, i.e. Chimaera and the Dipnoi, it 



FIG. 317. TRANSVERSE SECTION THROUGH THE VENTRAL PART OF THE 

 NOTOCHORD AND ADJOINING STRUCTURES OF AN ADVANCED SCYLLIUM EMBRYO 

 AT THE ROOT OF THE TAIL. 



Vb. cartilaginous sheath of the notochord ; ha. hasmal arch ; vp. process to which 

 the rib is articulated ; mcl. membrana elastica externa ; ch. notochord ; ao. aorta ; 

 . caudal vein. 



retains its primitive condition, except that in Chimaera there 

 are present delicate ossified rings more numerous than the 

 arches ; while in the Notidani, Laemargi and Echinorhini the 



