58 



CLASS PISCES. 



and for that we refer the reader to the account of the different 

 sub-classes. We must content ourselves here with an account 

 of its more general features, to which it is desirable to call 

 attention. 



The notochord forms the 

 basis of the axial skeleton. It 

 always persists in the adult, 

 though it is generally consider- 

 ably reduced. Its longitudinal 

 extent is from the pituitary 

 fossa of the skull in front to 

 the end of the vertebral column 

 behind. The notochord itself 

 rarely forms an important 

 element of the axial skeleton 

 of the adult. Its supporting 

 function — so conspicuous a 

 feature in Amphioxus — is taken 

 over by its sheath and by 

 cartilaginous structures de- 

 veloped around its sheatn. 

 In the trunk these structures 

 are, with few exceptions, seg- 

 mented, and constitute the 

 vertebral column ; in the skull 

 they are not segmented, and 

 are known in the embryo as 

 tlie parachordal cartilages. 



At an early stage in the embryo 



a well defined structureless sheath 



is formed round the notochord. 



This is called the memhrana elastica 



interna. A little later a second 



sheath is formed round this. This 



outer sheath, also structureless, is 



knowTi as the memhrana elastica 



externa. Cells of the skeletogenous mesoblastic layer, which surrounds 



the notochord, appear now to penetrate the elastica externa and invade 



the elastica interna (Fig. ,S3), which thus becomes nucleated. 



r 



The chordal sheath sometimes remains as a continuous 

 structure (Sturgeon, Dipnoi, etc.), but as a rule becomes seg- 



Fro. 33. — Transverse Sectioa tlirough 

 the vertebral column of an advanced 

 embryo of Scyllium in the caudal 

 region ; na skeletogenous tissue of 

 neural arch, ha of haemal arch ; ch 

 notochord ; sh notochordal sheath, 

 which has acquired nuclei (elastica 

 interna) ; ne outer cliordal sheath 

 (elastica externa) (After Balfour). 



