IX 



SKELETON 



315 



(dorsal — Fig. 131, C, n.a) and the haemal arches (ventral — Fig. 131, C, h.a) 

 respectively. The appearance of these blocks of cartilage is followed by 

 the occurrence of a remarkable process by which the whole secondary 

 sheath of the notochord becomes converted into a cylinder of cartilage. 

 Certain cells of the arch rudiments assume an amoeboid character, 

 forsake their original position, creep towards the primary sheath, bore 

 their way through it, apparently by the secretion of a digestive ferment, 

 andjfinding themselves in the secondary sheath (Fig. 131, Q,,c.c), distribute 



■Zl.<l. 



C.C. 



n.e. 



h.a. 



Fig. 131. 



Diagrammatic transverse sections through the notochord of a Dogfish {post-anal region) at 

 three different stages of development. A, Dorsal aorta (caudal artery) ; c.c, cartilage cells ; 

 c.t, connective tissue arches ; h.a, cartilaginous haemal arch rudiment ; n.a, cartilaginous neural arch 

 rudiment; «.e, uotochordal epithelium; s.l, primary sheath; s.II, secondary sheath; v, caudal 

 vein. 



themselves throughout its substance and there settle down — re-assuming 

 the character of typical cartilage cells and in turn so influencing the 

 substance of the sheath as to cause it to take on the typical, characteristics 

 of cartilage matrix. By this remarkable immigration of cartilage cells 

 from the neural arches the whole substance of the secondary sheath 

 becomes converted into cartilage. 



The wall of this at first homogeneous cylinder of cartilage becomes 

 thickened at regular intervals, the ring-shaped thickened portions growing 

 inwards so as to constrict the notochord which fills the interior of the 

 cylinder until in the centre of the thickening the notochord is reduced 



