48 



COMPARATIVE ANATOMY 



A delicate primary sheath is present and is surrounded by con- 

 nective tissue which is continuous with that enclosing the neural 

 canal and separating the muscular segments or myotomes. 



Cyclostomes. In these, as in all the true Fishes, only two 

 regions can be distinguished in the vertebral axis, a trunk- or pre- 

 caudal region, and a caudal region. An advance on the primitive 

 condition in Amphioxus is seen in the development of a thick 

 secondary sheath and, at any rate in the caudal region, of 

 cartilaginous elements : in the adult Petromyzon these are present 

 all along the notochord in the form of rudimentary neural arches, 

 which, however, do not meet above the spinal cord (cf. Fig. 34, B), 

 and of which there are two pairs to each muscular segment 



Tc 



FIG. 35. PORTION OF THE VERTEISRAL COLUMN OF Polyodon. Side view. 



FIG. 36. TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Acipenser 

 ruthenus (in the anterior part of the body). 



Ao, aorta; C, notochord ; Ee, primary, and Cs, secondary sheath of the noto- 

 chord ; EL, longitudinal elastic band ; Fo, median ingrowths of the lo wet- 

 arches enclosing the aorta ventrally ; Tc, intercalary pieces (inter-dorsal and 

 inter-ventral); M, spinal cord; Ob, upper arch (basi-dorsal) ; P, pia 

 mater ; P,s, neural spine ; SS, skeletogenous layer ; Ub, lower arch (basi- 

 ventral ; Z, "basal stiimps" of the lower arches. 



FIG. 37. PORTION OF THE VERTEBRAL COLUMN OF Protopterus. Side view. 

 C, notochord ; DF, neural spine ; FS, fin-ray ; FT, interspinous bone. 



or myotome (cf. Elasmobranchs). In the tail haemal arches, enclos- 

 ing the caudal aorta and vein, are also present, and fusion of the 

 cartilaginous elements occurs. 



Fishes. To the condition found in Cyclostomes, that seen in 

 the Cartilaginous Ganoids, Holocepliali, and Dipnoi, is directly 

 comparable : the notochord is persistent, no centra being formed, 

 as was also the case in the most primitive Palaeozoic Elasmo- 

 branchs, and thus the metameric character of the skeletal axis 

 is only seen in the arches (Figs. 35, 36, and 37). In the 

 Holocephali and Dipnoi, however, the thick secondary sheath 

 encloses cartilage cells amongst its fibres. In Chimasra narrow 



