62 COMPARATIVE ANATOMY. 



regions of the skull which are only membranous. 1 Again in other 

 cases, 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 come 

 under the category of the dermal skeleton and, as already men- 

 tioned with regard to the latter, are to be looked upon as originat- 

 ing genetically (Amphibia, Fishes) or phylogenetically (Amniota) 

 in connection with tooth-structures. In this manner, the bones 

 of the mouth-cavity of Fishes and Amphibians, for instance, still 

 arise, and this will not surprise us when we remember that the 

 epithelium of the oral cavity is formed by invagination of the 

 outer skin. 



This mode of origin of the first skull-bones appears 

 to be the oldest or 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 cal- 

 careous matter directly in the connective-tissue layer, without 

 giving rise to tooth-structures (e.g. in all investing bones, those, 

 for instance, of the roof of the skull of all Vertebrates from the 

 Amphibia to the Mammalia) : this may be looked upon as an 

 abbreviated development. 



The phylogenetically younger endochondral bones appear first in the 

 Anura and onwards, though in Urodeles only the perichondral mode of origin 

 is seen, and even in Anura this mode occurs largely. Not unfrequently, 

 endochondral bones and investing bones come into apposition, and fuse together. 

 Thus it may happen 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. 



The following lists give a summary of the most important 

 bones according to their different relations to the skull. 



I. Bones of the Mouth Cavity (partly lying within it, 

 partly bounding it on the outer side). 



1. Parasphenoid. 



2. Vomer. 



3. Premaxilla. 



4. Maxilla. 



INVESTING 

 BONES. 



5. Jugal. 



6. Quadratojugal (in part). 



7. Dentary. 



8. Splenial. 



9. Angular. 



10. Supra-angular. 



11. Coronoid. 



12. Palatine. 



13. Pterygoid. 



1 The different varieties of ossification may be conveniently classified as follows : 



I- Membrane Bpnes. () Dermostoses ossifications of the derma; (b) 

 Parostoses ossifications of thu looser subcutaneous tissue; (c) Ectostoses 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 to 



II- Cartilage Bones, which may, however, also be formed independently, a 

 bony deposit taking place within the cartilage itself (endostosis). 



