CHAPTER XVIII. 



THE NOTOCHORD, THE VERTEBRAL COLUMN, THE 

 RIBS AND THE STERNUM. 



Introduction. 



Amongst the products of that part of the mesoblast which con- 

 stitutes the connective tissue of the body special prominence must 

 be given to the skeleton of the Vertebrata, from its importance in 

 relation to numerous phylogenetic and morphological problems. 



The development of the skeleton is however so large a subject 

 that it cannot be satisfactorily dealt with except in a special treatise 

 devoted to it ; and the following description must be regarded as a 

 mere sketch, from which detail has been as far as possible excluded. 



In the lowest Chordata the sole structure present, which deserves 

 to be called a skeleton, is the notochord. Although the notochord 

 often persists as an important organ in the true Vertebrata, yet there 

 are always added to it various skeletal structures developed in the 

 mesoblast. Before entering into a systematic description of these, it 

 will be convenient to say a few words as to the general characters of 

 the skeleton. 



Two elements, distinct both in their genesis and structure, are 

 to be recognized in the skeleton. The one, forming the true 

 primitive internal skeleton or endoskeleton, is imbedded within the 

 muscles and is originally formed in cartilage. In many instances it 

 retains a cartilaginous consistency through life, but in the majority 

 of cases it becomes gradually ossified, and converted into true bone. 

 Bones so formed are known as cartilage bones. 



The other element is originally formed by the fusion of the 

 ossified bases of the dermal placoid scales already described in Chap- 

 ter XIV., or by the fusion of the ossified bases of teeth situated in the 

 mucous membrane of the mouth. In both instances the plates of bone 

 so formed may lose the teeth or spines with which they were in the 

 first instance covered, either by absorption in the individual, or phy- 

 logenetically by their gradually ceasing to be developed. The plates 

 of bone, which originated by the above process, become in higher 

 types directly developed in the connective tissue beneath the skin ; 



