Embryonal 

 cartilage 



DEVELOPMENT OF BONE. 47 



large extent absorbed and contributes only a small part of the mature 

 skeleton. The ear y marrow-cavity, including all its ramifications between 

 the trabecute, is filled with the young marrow-tissue; the latter eives rise 

 to the red marroxy that for a time fills all the bones and later occupies the 

 spongy tissue chiefly within the axial skeleton. --U may be emphasized that 

 the process sometimes spoken of as the "ossification of cartilage" is really 

 a substitution of osseous tissue for cartilage and that, even in the endo- 

 cnondral mode of formation, cartilage is never directly converted into bone 

 Ossification within the epiphytes, which usually does not begin until 

 some months after birth, repeats in its essential features the details of 

 mtracartilaginous bone-formation as seen in the development of the shaft 

 After the establishment of the primary 

 marrow-cavity and the surrounding 

 spongy bone, ossification extends in 

 two directions towards the periphery 

 and towards the adjacent end of the 

 shaft. As this process progresses, the 

 layer of cartilage between the central 

 spongy bone and the free surface, on 

 the one hand, and between the central 

 spongy bone of the epiphysis and that 

 of the shaft, on the other, is gradually 

 reduced until in places it entirely dis- 

 appears. Over the areas which cor- 

 respond to the later joint-surfaces, 

 the cartilage persists and becomes the 

 articular cartilage covering the ends 

 of the bone. With the final absorption 

 of the plates separating the epiphyses 

 from the shaft, the osseous tissue 

 composing the segments becomes con- 

 tinuous, "bony union" being then 

 accomplished. 



Subperiosteal Bone. It is evident 

 from the foregoing account of the 

 development of bone within cartilage 

 that the true bone-producing elements 

 are contributed by the periosteum 

 when the latter sends its processes into 

 the ossific centre within the cartilage. 

 The distinction, therefore, between intracartilaginous and subperiosteal bone 

 is one of situation rather than of inherent difference, as in the production 

 of both the osteoblasts are the active agents and the essential features are 

 identical. Since in the development of subperiosteal or perichondral bone 

 the changes within cartilage do not come into account, the details are less 

 complicated and concern primarily only a formative process. 



The young periosteum, it will be recalled, consists of an outer compact 

 fibrous layer and an inner loose osteogenetic layer. The latter is rich in 

 blood-vessels and contains numerous embryonal connective tissue cells and 

 delicate strands of fibres. Some of these cells become the osteoblasts and 

 as such are arranged along the fibrillae, about which the bone-matrix is 

 deposited through the influence of the cells. x The osseous trabecuUne, formed 

 in this manner beneath the periosteum, increase in length by the addition of 



Endochondral 



spongy bone 



Zone of 



calcification 



Embryonal 

 cartilage 



FIG. 60 Longitudinal section of p 

 foetus of five months. X 23 



