INTRODUCTION 7 



Cartilage Bone. — Cartilage persists as such in adult Cyclostomes 

 and Elasmobranchs and in some parts of all Vertebrates, the amount 

 remaining being less in the higher groups. Usually more or less of 

 it is converted into cartilage bone (autostoses) . This occurs in 

 two ways. In the lower Vertebrates perichondria! ossification 

 predominates, the inner cells of the perichondrium becoming bone- 

 formers (osteoblasts) which lay down bone on the outer surface of 

 the cartilage, the process gradually invading the deeper parts. In 

 the higher Vertebrates endochondral ossification prevails. Blood 

 vessels from the perichondrium invade the cartilage, dissolving the 

 matrix and setting its cells free, these now changing to osteoblasts 

 which arrange themselves around the blood vessels, and secrete the 

 ossein and lime salts of bone. With either type of ossification the 

 perichondrium becomes the periosteum. 



Membrane bones (allostoses) arise without a cartilage stage, the 

 connective tissue cells secreting ossein and lime directly, some cells 

 becoming osteoblasts, those surrounding them 

 forming the periosteum. Many membrane 

 bones, especially in the skull, are regarded as 

 having arisen by the fusion of the basal plates 

 of placoid scales or their homologues, teeth, the 

 fusion of such parts being well marked in the fjg. 6.— Vomer of 

 growth of dermal bones of Ganoids (Polypterus, Ambiystoma (Hertwig) 



. , showing bone formed by 



Acipenser). The participation of teeth occurs fusion of bases of teeth. 

 in the oral region. The cavity of the mouth 



is largely lined by an involution (stomodeum) of the skin which 

 has carried with it scales which have become teeth, the develop- 

 ment of teeth and placoid scales being practically identical. 

 In many animals it is evident that at least some of the bones 

 near the oral surface have been formed by the fusion of the bases of 

 teeth (fig. 6). In the higher groups this bone formation has been 

 emancipated from the teeth, the latter not appearing until after 

 the bones are well outhned. There are also membrane bones which 

 arise as ossifications of ligaments, like the patella of mammals and 

 the ligamentary bones in- the 'drumstick" of the turkey. A third 

 type of membrane bone occurs in fishes where the lateral line organs 

 are surrounded by bony tubes (fig. loo), especially those parts of 

 the canal around the eye. 



