THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 171 



arrange themselves in single, fairly regular rows along the bundles of calcified 

 fibers. The differentiated cells are known as osteoblasts (bone formers), and the 

 whole tissue is now known as osteo genetic tissue. Under the influence of the osteo- 

 blasts a thin layer of calcium salts is deposited between the osteoblasts and the 

 calcified fibers. In this way the first true bone is formed, and the calcification 

 center becomes an ossification center. Successive layers or lamellae of calcium 

 salts are laid down and some of the osteoblasts become enclosed between the 

 lamellae to form the bone cells (Figs. 151 and 152). The spaces in which the bone 

 cells lie are the lacuna. At the same time the fibers also are enclosed within the 

 bone and give it its characteristic fibrous structure (Fig. 152). 



Such a process results in the formation of irregular, anastomosing trabeculae 

 of bone. The spaces among the trabeculae are known as primary marrow 



Osteogenetis 



tissue Osteoclast Lacunae 



Bone 



_ , - ~-J 



Calcified fibers Osteoblasts 



FIG. 152. From vertical section through parietal bone of human foetus of 4 months. 

 Bone cells not shown in lacunae. (Intramembranous ossification.) 



spaces and contain osteogenetic tissue (Fig. 151). This type of bone, consisting 

 of irregular, anastomosing trabeculae and enclosed marrow spaces, is known as 

 spongy bone. The spongy bone thus formed is covered on its outer side by a 

 layer of connective tissue which from its position is called the periosteum 

 (Fig. 151), and which represents a part of the original embryonic connective 

 tissue membrane in which the bone was laid down. During its development 

 the periosteum becomes an exceedingly dense fibrous membrane which is closely 

 applied to the surface of the bone. 



In a growing embryo, provision must be made for increase in the size of the 

 cranial cavity to accommodate the growing brain. This is accomplished in 

 the following manner : On the inner surface of the newly formed bone, large 

 multinuclear cells appear, which are known as osteoclasts (bone destroyers). 

 The osteoclasts are unusually large cells with a large number of nuclei and 

 abundant cytoplasm, and in sections can be seen lying in depressions in the 



