THE SKELETON 



321 



similar lines. In the Eeptiles and also in Birds the palato-ptery- 

 goid outgrowth is again reduced in size — in correlation with the 

 fact that in the Tetrapod Vertebrates the tooth-bearing function of 

 the original upper jaw or palato-pterygoid bar has been taken over 

 by the secondary upper jaw composed of bones such as the maxilla 

 and premaxilla. 



BONY OR OSSEOUS SKELETON 



Bone, like cartilage, is a modified connective tissue. In its 

 typical form it differs from cartilage in the facts, that its matrix 

 yields on being boiled a larger proportion of gelatine, that the 

 matrix is rendered rigid by being strongly calcined, and that the 

 cytoplasm projects as slender branching processes which ramify 



Fig. 159. —Skeleton of visceral arches and pectoral girdle of 20\5 mm. embryo of Spinax. 



(After Brans, 1906.) 



£1, £5. branchial arches ; Hy, hyoid ; I, labial cartilage ; M, mandibular arch ; p, palato-pterygoid 

 bar ; p.f, rudiment of pectoral fin ; p.g, pectoral girdle ; Q, knob for attachment to trabecular region 

 of skull. 



through the matrix and are commonly continued into those of other 

 cells. Many different varieties of bony tissue exist. In ordinary 

 bone the cell elements are completely surrounded by the calcified 

 matrix. On the other hand some of the cells may have the main 

 part of their cell-body outside the calcified mass, only a slender 

 prolongation being surrounded by it (Bones of Lepidosteus and 

 AmicL). Or this peculiarity may apply to all the cells (Dentine of 

 higher Vertebrates) or finally no cells or parts of cells are enclosed 

 within the hard matrix— as is often the case in early stages of 

 development and as occurs in the adult condition in many Teleostean 

 fishes. 



Probably the' most archaic type of bony skeleton in existing 

 Vertebrates is seen in the Placoid scales of the Elasmobranchs and 

 consequently the mode of development of these will logically fall to 

 be considered first. 



The appearance of the scale is foreshadowed by a localized 

 condensation of the dermal connective tissue immediately beneath 



VOL. II Y 



