VERTEBRAL COLUMN. 



35 



To the condition found in Petromyzon, that seen in the 

 Cartilaginous Ganoids, ChimaBrse, and Dipnoi, is directly 

 connected, inasmuch as the metameric character is mainly indicated 

 by upper or dorsal arches. 



Ub 



FIG. 21. PORTION OF THE VERTEBRAL COLUMN OF Spatularia. (Side view.) 



FIG. 22. TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Acipenser 

 ruthenus (in the anterior part of the body). 



Pa, spinous process ; EL, longitudinal elastic band ; SS, skeletogenous layer ; Ob, 

 upper arch ; M t spinal cord ; P, pia mater ; Ic, intercalary pieces ; Cs, noto- 

 chordal sheath ; C, notochord ; Ee, elastica externa ; Ub, lower arch ; Ao, aorta ; 

 Fo, median parts of the lower arches, which enclose the aorta ventrally ; Z, basal 

 processes of the lower arches. 



The strong, concentrically-layered notochordal sheath (Fig. 22, 

 Cs) here plays the part of vertebraa, and is surrounded by a fibrous 

 skeletogenous layer (Fig. 22, $#), in which paired dorsal and 

 ventral cartilages become developed. The former give rise to 

 the above-mentioned upper or neural arches, the latter to the 

 lower or haemal arches (Figs. 21,22, Ob, Ub]. In the caudal region 



FIG. 23. PORTION OF THE VERTEBRAL COLUMN OF Protopterus. (Side view.) 

 C, notochord ; DF, spinous process ; FT, interspinous bone ; FS, fin-ray. 



the latter enclose the aorta and caudal vein ; further forwards the 

 cartilages do not meet in the middle line below, and consequently 

 the lower arches end on either side in a laterally-directed cartila- 

 ginous projection, or " basal process," which may develop an 



D 2 



