THE ENDOSKELETON 127 



s the oldest, that part becomes the most restricted, often com- 

 pletely severing the notochord at this point, or intra-verte- 

 brally; while the notochord is retained at practically its original 

 caliber at the newest edges of the ring, or inter-vertebrally. 

 The completed ring, which forms the body or centrum of the 

 vertebra, is cylindrical, with concave ends like the interior of 

 conical cups. Such vertebrae are called cwnphiccelous (=both 

 ends hollow), and the hollows of the adjacent vertebrae enclose 

 masses of notochord in the form of two cones, placed base to 

 base. 



Although the above sketch is, in a way, hypothetical, many 

 of the stages described actually occur as the adult condition in 

 various fishes, especially ganoids, and the final condition is 

 exactly shown by the teleosts, as one may have frequent occa- 

 sion to observe. That the evolution of the vertebral column 

 up to this point has been somewhat after the plan here given, 

 may be generally conceded, and it is hoped that important 

 links in the history may be discovered in the field of palaeontol- 

 ogy, which has already furnished us so many valuable records 

 and bridged so many gaps. 



Above the fish the development of the vertebral column has 

 been, not so much in the acquirement of new elements, as in 

 the regional modification of those already possessed. This is 

 strikingly shown by the comparison of the vertebral column 

 of a fish with that of a reptile or mammal ; in the first of these 

 the vertebrae are all very much alike, while in the second they 

 are differentiated into successive groups, and in cases in which 

 this differentiation has reached its extreme each vertebra may 

 be sufficiently unlike the rest to be identified by the anatomist 

 when isolated, a feat impossible in the former case. 



This regional differentiation is due chiefly though indirectly, 

 to the change of environment from water to land, a change 

 which necessitates the replacement of soft and weak fins by 

 two pairs of firm limbs, and substitutes for the evenly dis- 

 tributed buoyancy of the water a fixed support at two points, 

 the shoulder and hip girdles. In the first land animals the 

 limbs were small and weak, and progress was attained through 



