NOTOCHORD AND VERTEBRAL COLUMN. 543 



converted into true bone. Bones so formed are known as 

 cartilage bones. 



The other element is originally formed by the fusion of the 

 ossified bases of the dermal placoid scales already described in 

 Chapter xiv., or by the fusion of the ossified bases of teeth 

 situated in the mucous membrane of the mouth. In both 

 instances the plates of bone so formed may lose the teeth or 

 spines with which they were in the first instance covered, either 

 by absorption in the individual, or phylogenetically by their 

 gradually ceasing to be developed. The plates of bone, which 

 originated by the above process, become in higher types directly 

 developed in the connective tissue beneath the skin ; and 

 gradually acquire a deeper situation, and are finally so inti- 

 mately interlocked with parts of the true internal skeleton, that 

 the two sets of elements can only be distinguished by the fact 

 of the one set ossifying in cartilage and the other in membrane. 



It seems probable that in the Reptilia, and possibly the 

 extinct Amphibia, dermal bones have originated in the skin 

 without the intervention of superjacent spinous structures. 



In cases where a membrane bone, as the dermal ossifica- 

 tions are usually called, overlies a part of the cartilage, it may 

 set up ossification in the latter, and the cartilage bone and mem- 

 brane bone may become so intimately fused as to be quite in- 

 separable. It seems probable that in cases of this kind the 

 compound bone may in the course of further evolution entirely 

 lose either its cartilaginous element or its membranous element; 

 so that cases occasionally occur where the development of a 

 bone ceases to be an absolutely safe guide to its evolution. 



As to the processes which take place in the ossification of 

 cartilage there is still much to be made out. Two processes are 

 often distinguished, viz. (i) a process known as ectostosis, in 

 which the ossification takes place in the perichondrium, and 

 either simply surrounds or gradually replaces the cartilage, and 

 (2) a process known as endostosis, where the ossification actually 

 takes place between the cartilage cells. It seems probable 

 however (Gegenbaur, Vrolik) that there is no sharp line to be 

 drawn between these two processes ; but that the ossification 

 almost always starts from the perichondrium. In the higher 

 types, as a rule, the vessels of the perichondrium extend into 



