THE VERTEBKATA OR VERTEBRATES. 



together, as do the parts of a broken bone, but they are 

 merely cemented, or, as it were, soldered together by new- 

 ly-added portions of the shell-substance. 



As to the origin of the skeleton, it may be stated that it 

 is not in any case a primary formation ; that is, it does not 

 at once appear as a bony structure in any animal ; but, on 

 the contrary, it is the result of successive changes which 

 take place in pre-existing tissues. 



The central basis of the skeleton appears, in the embryo 

 of every vertebrate animal, as a cylindrical, fibrous sheath 

 filled with simple cells containing a jelly-like substance ; 

 this constitutes what anatomists call the chorda-dorsal is or 

 notochord, and it is from this notochord that the centrums, 

 that is, the main bodies of the vertebrae, are developed. 



And, in general terms, it may be stated that in the 

 place of every bone in a vertebrate animal, there is at first 

 only a clear soft substance. In the development and 

 growth of the animal this substance becomes progressively 

 more and more firm, acquiring, in turn, the nature ot 

 membrane or ligament, and then, perhaps, of cartilage ; and 

 then ossification or real bone-making next ensues.* The 

 ossification, or the introduction of the earthy particles, that 



* The chemical composition of the skeleton in four prominent kinds 

 of vertebrate animals, is shown in the following table from Owen : 



