3i6 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



to a comparatively thin thread. In the skate it becomes interrupted 

 entirely. The thickening of the cartilaginous wall of the cylinder dies 

 away anteriorly and posteriorly and is separated from the next thickened 

 portion by a ring of unthickened cartilage which gradually loses its original 

 character and becomes converted into tough fibrous tissue. In this way 

 the originally continuous cylinder of cartilage becomes segmented up 

 into a series of pieces known as vertebral centra, bound together by strong 

 intervertebral ligaments. It will be understood that each centrum has 

 externally the form of a short cylinder, and that it contains two . deep 



Fig. T32. 

 Transverse section through front portion of tail region of Acanthias. A, Dorsal aorta (caudal 

 artery) ; h.a, haemal arch ; m, myotomes separated by thin septa of connective tissue ; n, noto- 

 chord ; n.a, neural arch ; s.T, primary sheath ; s.II, secondary sheath ; s.c, spinal cord ; sk, skin ; 

 V, caudal vein. 



cavities — one anterior and one posterior — in the form of two cones 

 arranged apex to apex and opening into one another at the point where 

 the two apices meet. Such a vertebral centrum with a conical cavity 

 at each end is termed amphicoelous and this form of centrum is charac- 

 teristic of all ordinary fishes. In the fresh condition the conical cavities 

 are not empty but are filled by the notochord which traverses them. 



At their distal ends the neural arch rudiments gradually extend until 

 they meet in the mesial plane over the spinal cord (Fig. 132) and the 

 apical portion so formed continues its outward growth to form a neural 

 spine. 



An interesting point to be noticed in the Dogfish and in various other 



