DEVELOPMENT OF THE BONES. 851 



Thus in the cartilaginous cranium the vault is entirely wanting, 

 and almost all the lateral portions, as well as nearly all of what 

 afterwards becomes the facial bones. The parts not formed of car- 

 tilage are, however, closed .by a fibrous membrane, so that the cra- 

 nium is nevertheless at this time complete. It is also in this mem- 

 brane that the secondary bones are subsequently developed. 



The changes which occur in the primordial cartilaginous skeleton 

 are, therefore, of three kinds : 1. Some portions subsequently dis- 

 appear altogether, as already stated. 2. Other portions undergo 

 subsequent development with the rest of the skeleton, constituting 

 the permanent cartilages of the nose, joints, symphyses, and syn- 

 chondroses. 3. The third and greater part finally becomes ossified, 

 forming all the bones of the trunk and extremities, and a great part 

 of those of the cranium. It is this portion the ossifying cartilages 

 whose changes are to be described here. 



The general description of the ossification of the cartilages is as 

 follows : At one or more points in their interior* calcareous matter 

 begins to be deposited, simultaneously with a change in the elements 

 of the cartilage. The latter, in most cases, ceases to grow in one 

 direction while this change is going on, and is therefore soon en- 

 tirely converted into bone; while in other directions its growth 

 continues, so that a new cartilaginous material is offered for the 

 progressive increase of the bone. When the bone has attained to 

 its ultimate length, the cartilage becomes completely ossified, and 

 ceases to be developed, its perichondrium now being a periosteum. 

 The diameter of the bone is, however, still increased by the forma- 

 tion of concentric lamina (the fundamental laminae, p. 328) under- 

 neath the periosteum, from the blood-plasma in the periosteal vessels. 



The minute changes in the ossifying cartilage are next to be de- 

 scribed, and Kolliker's view of this difficult subject will be adopted 

 as the most satisfactory. 



It should, however, be premised that the cancellated tissue alone 

 is developed from the primordial cartilages, the compact tissue being 

 derived from another source. When, therefore, the cartilages have 

 'become ossified, the bones thus formed all consist entirely of cancel- 

 lated tissue. The following description, therefore, of the ossification 

 of the cartilages is the history of the development of the cancellated 

 lone-substance, wherever found : 



I. Before ossification actually commences, vessels begin to pene- 

 trate the ossifying cartilages. They appear from and after the 



