296 



HISTOGENESIS 



Howship's lacuna. The bone lamellae of the central portion of the membrane 

 bone are gradually resorbed and this portion of the bone is of a spongy texture. 

 Some time after birth, compact bone lamellae are laid down by the inner osteo- 

 blast cells of the periosteum. In the case of flat bones, compact inner and outer 

 plates or tables are thus developed 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 



Periosteum 



Osteoblas1\ 



tv' 2 



I* 



-F?ji*3?fSG%* 



~* ~- ^ e * 



wC^f * 

 ^> ffi -<* f 



J3one cell 



Bone I \ \0sfsocta.sf" 



' Fibrills in bone matrix 



FIG. 286. Two stages in the development of bone : A , section through the occipital cartilage of a 

 20 mm. pig embryo (Mall); B, section through the periosteum and bone lamellae of the mandible from a 

 65 mm. human embryo. X 325. 



the red bone marrow. The ossification of membrane bones begins at the middle 

 of the bone and proceeds in all directions from this primary center. 



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

 formed cartilage and its surrounding membrane, the perichondrium (Fig. 288). 

 Bone tissue is developed as in membrane bones save that the cartilage is first 

 destroyed and the new bone tissue develops (i) in and (2) about it. In the first 

 case, the process is known as endochondral bone formation. In the second case, 

 it is known as perichondral or periosteal bone formation. 



