DEVELOPMENT OF BONE. 



43 



tissue; such are said, therefore, to be membrane bones and their formation is 

 by intramembranous development. 



Endochondral Bone-Development. The process by which bones 

 preceded by cartilage are formed, known as the endochondral mode 

 includes two distinct, although closely related, series of changes. The one 

 results in the production of osseous tissue within the mass of cartilage the 

 intracartilaginous bone, the other in the production of bone outside the 

 cartilage and beneath the periosteum, the subperiosttal bone. Both take 

 part in the formation of the completed bone, although their contributions 

 to the final result are not only 

 unequal, but vary with differ- 

 ent types of bones. 



The greater part of the 

 bone formed within the carti- 

 lage undergoes absorption, 

 the spongy substance within 

 the ends of the long and the 

 bodies of the irregular bones 

 chiefly representing the per- 

 sisting contribution of the in- 

 tracartilaginous bone. Even 

 when the intracartilaginous 

 changes are conspicuous, as 

 in the development of the hu- 

 merus, femur and other long 

 bones, the important compact 

 substance is the product of 

 the periosteal connective tis- 

 sue and genetically resem- 

 bles intramembranous bone. 

 Although the formative proc- 

 esses of both kinds of bone 

 proceed coincidently and are 

 closely related, as a matter of 

 convenience they will be , vc ... 

 described separately and as ^ !v 'V-;^'>' 

 occurring in the development 

 of a typical long bone. 



Intracartilaginous Bone. 

 The primary cartilage, 



formed by the proliferation and condensation of the mesenchymal tissue, 

 gradually assumes the characteristics of embryonal cartilage, which by the 

 end of the second month of foetal life maps out the principal segments of 

 the skeleton. These segments are covered by an immature form of peri- 

 chondrium, or primary periosteum, from which arise the elements actively 

 engaged in the production of the bone-tissue. The primary periosteum 

 consists of a compact outer fibrous and a loose inner osteogenetic layer, the 

 latter being rich in cells and delicate intercellular fibres. 



The initial changes within the cartilage appear at points known as 

 centres of ossification, which in the long bones are situated about the middle 

 of the future shaft. These early changes involve both cells and matrix, 

 whicn exhibit conspicuous increase in size and amount respectively. The 

 cartilage-cells become larger and more vesicular (Fig. 56) and encroach upon 



Embryonal 

 'cartilage 



Cartilage-cells 

 becoming enlarged 

 and regrouped 



Enlarged cartilage- 

 cells at centre of 

 ossification 



Periosteum 



FIG. 56. Section of tarsal bone of fojtal sheep, showing centre 

 of ossification. X 50. 



