204 



THE ORGANS 



formers.^' Under the influence of these osteoblasts a thin plate of bone is formed 

 between themselves and the calcified fibres. This plate of bone at first contains 

 no cells, but as the lamella of bone grows in thickness, the layer of osteoblasts 

 becomes completely enclosed by bone. The osteoblasts are thus transformed 

 into bone cells (Fig. 122), the spaces in which they lie becoming bone lacunce. The 

 bone cell is thus seen to be derived from the embryonic connective-tissue cell, 

 the osteoblast being an intermediate stage in its development. In this way 

 irregular anastomosing trabeculae of bone are formed enclosing spaces (Fig. 

 121). The bony trabeculas at first contain remains of calcified connective- 

 tissue fibres, while the spaces, which are known as primary marrow spaces, con- 

 tain blood-vessels, osteogenetic tissue, and developing marrow. The osteo- 

 blasts ultimately disappear and the spaces are then occupied by blood-vessels 

 and marrow. The connective-tissue membrane has now been 'transformed into 

 cancellous or spongy boiie (Fig. 115). 



a b 



Fig. 122. — Intramembranous Bone Development. Vertical .section through parietal 

 bone of human foetus. X350. (Technic i, p. 208.) a, Osteoblasts; b, calcified 

 fibres; c, osteogenetic tissue; d, osteoclast lying in Howship's lacuna; e, bone lacunae; 

 /, bone. 



The bone thus formed is covered on its outer surface by a layer of con- 

 nective tissue, a part of the membrane in which the bone was formed, but 

 which from its position is now known as the periosteum, or, in the case of the 

 cranial bones, as the peri- or epicranium (Fig. 121). 



In this form of bone development, occurring as it does in the bones of the 

 skull, provision must be made for increase in the size of the cranial cavity to ac- 

 commodate the growing brain. This is accomplished in the following manner: 

 Along the surface of the bone, directed toward the brain, large multinuclear 

 cells — osteoclasts or ^'bone breakers'" — make their appearance (Fig. 122). The 

 origin of these cells is not clear. Similar cells are conspici' "•" '^i^mpnts of adult, 

 marrow. They have been variously described as der cocytes 



from osteoblasts, or directly from the connective- tissue tt:Us. theory 



holds that they are derived by a process of budding froi al cells, 



which form the walls of the capillaries. These osteoch possess 



the power of breaking down bone. They are found mai ■ ner sur- 



face, and can be seen lying in little depressions — Howsh . 122) — 



