VERTEBRAL COLUMX 37 



mentary neural (dorsal) arches, which, liowever, do not meet above 

 the spinal cord. These cartilages, of which there are two pairs to 

 each muscular segment or myotome, correspond to the " intercalary 

 pieces" of Elasmobranchs (p. 38); they serve in the first instance 

 for the origin and insertion of the muscles, and at the same time 

 form a protection for the spinal cord. Neural spines also occur 

 in the middle of the axis, and in the caudal region hcernal 

 (ventral) arches enclosing the caudal aorta and vein, as well as 

 hcemal spines, are present, and fusion of the cartilaginous elements 

 occurs. 



To the condition found in Cyclostomes, that seen in the 

 Cartilaginous Ganoids, Holocephali, and Dipnoi is directly 

 connected, inasmuch as the metameric character of the skeletal axis 



Fill. 2'ii. — Portion op the Vertebral L'olumx of ,b>'(<H/V(/7V(. (Side view. ) 



Fig. 24. — Transverse Section of the Vertebral Colujin of Aciptuv-r 

 rutheiiux (in the anterior part of the bod}'). 



Pn, spinous process ; J<1L, longitudinal elastic band ; SS, skeletogenous layer ; Oli, 

 upper arch ; 3/, spinal cord ; P, pia mater ; Ic, intercalary pieces ; O, noto- 

 chord ; He, primary, and C'.s, secondary sheath of the notochord ; U1>, lowei' 

 arch ; Ao, aorta ; Po, median parts of the lower arches, which here enclcie 

 the aorta ventrally ; Z, basal processes of the lower arches. 



is essentially indicated by the neural arches. In the two groups last 

 mentioned, however, skeletogenous cells break through the primary 

 notochordal sheath (elastica) and so invade the thick secondary 

 sheath, which in consequence encloses cartilage cells amongst its 

 fibres. In Chimsera calcified rings are also developed in the central 

 part of the sheath : these are more numerous than the arches. 

 The latter are developed as paired dorsal and ventral cartilages : 

 they remain cartilaginous in the Cartilaginous Ganoids (Figs. 2S 

 and 24) and Holocephali, but become densely ossified in the Dipnoi 

 (Fig. 25). In the caudal region the heemal arches enclose the 

 caudal aorta and vein ; further forwards the cartilages do not meet 

 in the middle line below, and consequently the lower arches end 



