THE SKULL 71 



bones), and those which arise in connective-tissue, entirely inde- 

 "pendent of cartilage (jiierribrane- or investing -iones). But it must 

 be borne in mind that there is no hard and fast line between 

 these, and that histologically they are indistinguishable from one 

 . another. Bone is always phylogenetically formed oictside the 

 ■cartilage, and its first appearance within cartilage (as in the 

 Amniota more particularly) is to be looked upon as a secondary 

 condition.' Again, in other cases (e.g., in parts of the skeleton of 

 Elasmobranchs), true bones are not formed at all, there being 

 only a calcareous incrustation of the cartilage (calcified cartilage). 



The bones arising in the membranous regions of the skull 

 (including the perichondrium) primarily come under the category 

 of the dermal skeleton and, as already mentioned with regard to 

 the latter, are to be looked upon as originating phylogenetically 

 in connection with dermal denticles (p. 30). In this manner 

 the bones of the mouth-cavity of Fishes and Amphibians, for 

 instance, still arise : it must be. remembered that the epithelium 

 of the oral cavity is formed by invagination of the outer skin. 



Such a mode of origin of the first skull-bones appears to be the 

 oldest and most primitive, and it is most apparent in the lower 

 Vertebrates (Fishes). This holds good also for those cases in which 

 bones are formed merely by deposition of calcareous matter directly 

 in the connective-tissue layer, without giving rise to tooth-struc- 

 tures (e.g., all investing bones) — probably owing to an abbrevia- 

 tion of development. 



The following lists give a summary of the most important 

 bones according to their different relations to the skull. 



I. Investing Bones of the Mouth-Cavity (partly lying 

 within it, partly bounding it on the outer side). 



1. Parasphenoid. 



2. Vomer. 



3. PremaxilJa. 



4. Maxilla. 

 0. Jugal. 



6. Quadratojugal (in part). 



7. Dentary. 



1 The diflferent varieties of ossification may be conveniently classified as 

 follows : — 



I. " Membrane Bones." (f») Dermostoses — ossifications of the dermis ; (ft) 

 parostoses — ossifications of the looser subcutaneous tissue ; (c) ectostosen — ossifi- 

 cations of the inner layer of the fibrous investment (perichondrium) of a tract of 

 cartilage : these may extend into the latter, replacing it, and thus give rise 

 secondarily to 



II. " Cartilage Bones," {endo^tosea). 



It may, however, liappen that in the course of generations an investing bone 

 may take the place of a cartilage bone, and the formation of cartilage be entirely 

 suppressed and not repeated again ontogenetically. 



