448 MEMBRANE BONES AND CARTILAGE BONES. 



and gradually acquire a deeper situation, and are finally so intimately 

 interlocked with parts of the ti'ue 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 ossifications 

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

 ossification in the latter, and the cartilage bone and membrane bone 

 may become so intimately fused as to be quite inseparable. 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 distin- 

 guished, viz. (1) 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 the cartilage, 

 and the ossification takes place around these vessels within the carti- 

 lage; but in the lower types (Pisces, Amphibia) ossification is often 

 entirely confined to the perichondrium; and the cartilage is simply 

 absorbed. 



The regions where ossification first sets in are known as centres of 

 ossification ; and from these centres the ossification spreads outwards. 

 There may be one or more centres for a bone. 



The actual causes which in the first instance gave rise to particular 

 centres of ossification, or to the ossification of particular parts of the 

 cartilage, are but little understood ; nor have we as yet any satisfactory 

 criterion for determining the value to be attached to the number and 

 position of centres of ossification. In some instances such centres 

 appear to have an important morphological significance, and in other 

 instances they would seem to be determined by the size of the carti- 

 lage about to be ossified. 



Tbei-e is no doubt that the membrane bones and cartilage bones can 

 as a rule be easily distinguished by tlieir mode of development ; but it is 

 by no means certain that this is always the case. It is iiecessai'ily very 

 difficult to establish the homology between bones, which develop in one 

 type from membrane and in another type from cartilage ; but tliere are 

 without doubt certain instances in wliich tlie homolog}' between two bones 

 would be inihesitatinglv admitted were it not for the difference in their 



