CHAPTER XVIII. 



THE NOTOCHORD, THE VERTEBRAL COLUMN, THE 

 RIBS AND THE STERNUM. 



INTRODUCTION. 



AMONGST the products of that part of the mesoblast which 

 constitutes the connective tissue of the body special prominence 

 must be given to the skeleton of the Vertebrata, from its impor- 

 tance 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 



