J T ER TEBRA L COL UMN 



99 



Actinopterygii, the notochordal sheaths remain unbroken through- 

 out ontogeny, the vertebral centra being formed round them inter- 

 segmentally. Generally the vertebral rings, or half-rings, developed 

 in connection with the arches constrict the notochord, so as even to 

 obliterate it in the middle of each centrum (Fig. 94). 



In other fish, the Dipnoi, the mesoblastic cells of the skeleto- 

 genous layer at the base of the arches pierce the thin elastica 



a- 



l-'lG. CO. 



Scyllium canicula, L. Transverse section of the vertebral column of the tail of a young 

 embryo. A, the base of the neural arch in an older embryo ; B, of a later stage ; C, all magnified. 

 a, caudal artery ; c, cartilage ; c.i, 'elastica interna' or inner layer of the fibrous sheath ; el, 

 elastica externa ; f.s, fibrous sheath which becomes invaded by mesoblastic cells ; h.a, haemal 

 arch ; n.a, neural arch ; a.c, nerve-cord ; nt, notochord ; nt.cp, notochordal epithelium ; mi, 

 nuclei of mesoblastic cells passing through the broken elastica externa ; v, caudal vein. 



externa, and invade the fibrous sheath into which they migrate at 

 four points in its circumference in each segment. Here they give 

 rise to a small amount of cartilage ; without, however, forming dis- 

 tinct centra (Fig. 204). But in the Elasmobranchs the invasion is 

 much more thorough. Almost the whole thickness of the wide 

 sheath is formed of intrusive mesoblastic tissue in late stages. The 

 original fibrous sheath often remains next to the notochord as a 

 clear thin layer, the so-called elastica interna (Fig. 60). Both 



