60 GNATHOSTOMATA 



(p. 2), the primitive arrangement of both septa and muscles 

 becomes modified and obscured. 



The skeleton of the trunk first develops in connection with the 

 main notochordal axis and transverse septa : vertebrae are formed 

 between the-myotomes, where the septa meet the covering of the 

 notochord and neural canal and ribs are added where the same 

 septa intersect the horizontal septum and the coelomic wall 

 (Fig. 40). 



Passing now to the origin and finer structure of the endo- 

 skeleton, we find that it arises in the mesoblastic tissue, and takes 

 the form of cartilage. Cartilage is a tissue in which the cells secrete 

 a firm, compact elastic matrix, more or less homogeneous and free 

 from fibres, the chemical basis of which yields chondrin (Fig. 41). 

 The cells, which are generally oval and provided with fine com- 



Fio, 41. 



Section of articular cartilage of Mail, showing transition of cartilage-cells, a, to branching 

 connective-tissue cells, b. (From Quain's Anatomy, by permission of the Publishers.) 



municating branches, are distributed throughout the matrix, and 

 multiply by division. The matrix not being rigid, cartilage can 

 grow equally in all directions, i.e. in the three dimensions of space. 

 This is its most characteristic feature. Intermediate forms of tissue 

 are found between such typical * hyaline ' cartilage and fibrous 

 connective tissue. 



Bone is a form of connective tissue which develops in relation 

 to the endoskeleton later than cartilage. This is the case in both 

 phylogeny and ontogeny. In the higher vertebrates bone gradually 

 increases in importance as a skeletal tissue, and eventually almost 

 entirely supplants the cartilaginous framework on which it has been 

 moulded. There has been, so to speak, a struggle between cartilage 

 and bone, in which the latter has triumphed. 



In the majority of cases, then, bone has replaced cartilage, and, 

 as a matter of fact, we find that in ontogeny the bony skeleton is, 

 for the most part, actually preformed in cartilage. The advantage, 



