288 HISTOGENESIS 



is converted into a homogeneous bone matrix, which first takes the form of 

 spicules. Others view the fibrillated matrix as an intercellular product 

 and the bone matrix as an interfibrillar deposit. However this may be, 

 the spicules coalesce, form a network of bony plates, and constitute the 

 bone matrix upon the surface of which osteoblasts are arranged in a 

 single layer like the cells of an epithelium (Fig. 294 B). As the matrix 

 of the bone is laid down, osteoblasts become engulfed and form bone cells. 

 The bone cells are lodged in spaces termed lacuna. These are connected 

 by microscopic canals, the canaliculi, in which delicate cell processes 

 course. 



Ossification begins at the middle of the bone and proceeds in all 

 directions from this primary center. The plates of the spongy membrane 

 bone are formed about blood vessels as. centers. As the bone grows at 

 the periphery, the bone matrix is resorbed centrally. At this time, large, 

 multinculeate cells appear upon the surfaces of the bone matrix. These 

 cells are known as osteoclasts (bone destroyers). There is, however, no 

 positive evidence that the osteoclasts are active in dissolving the bone. 

 They may be interpreted also as degenerating, fused osteoblasts (Arey, 

 1920). The cavities in which they are frequently lodged are known as 

 Howship's lacunas. The bone lamellae of the central portion of the mem- 

 brane bone are gradually resorbed, and this portion of the bone is of a 

 spongy texture. Some time after birth, compact bone lamellag are laid 

 down by the osteoblast cells of the inner layer of the periosteum. In the 

 case of flat bones, compact inner and outer plates, or tables, are thus devel- 

 oped with spongy bone between them. The spaces in the spongy bone 

 are filled by derivatives of the mesenchyme : reticular tissue, blood vessels, 

 fat cells, and developing blood cells. These together constitute the red 

 bone marrow. 



Cartilage Bone. The form of the cartilage bone is determined by the 

 preformed cartilage and its surrounding membrane, the perichondrium 

 (Fig. 296). Bone tissue is developed as in' membrane bones, save that 

 the cartilage is first gradually destroyed and the new bone tissue develops: 

 (i) in, and (2) about it. In the first case, the process is known as endo- 

 chondral bone formation. In the second cas"e, it is known as perichondral, 

 or periosteal bone formation. 



Endochondral Bone Formation. The cartilage cells enlarge, become 

 arranged in characteristic rows, and lime is provisionally deposited in 

 the matrix (Fig. 295). The perichondrium becomes the periosteum. 

 From its inner or osteogenic layer, which is densely cellular, ingrowths 

 invade and resorb the cartilage and fill the primary marrow cavities. 

 The invading osteogenic tissue gives rise to osteoblasts and bone marrow. 

 The osteoblasts deposit bone directly upon persisting portions of the carti- 



