62 



THE CONNECTIVE TISSUES 



of matrix that unite to form trabeculse radiating in all directions. 

 (Fig. 34.) The osteoblasts surrounding such spicules resemble an 

 epithelial membrane and their continued activity results in growth 

 of the spicules in thickness and length. With increasing deposits of 

 matrix, some of the osteoblasts become surrounded and remain in 

 lacunae, in which location they are called osteocytes. Processes of 

 these imprisoned cells radiate through canaliculi in the matrix to 

 make contact with cytoplasmic processes of adjoining cells. After 

 the appearance and early activity of these ossification centers, the 

 mesenchyme surrounding the developing spongy plate condenses 



Fig. 34. — Photograph of a developing membranous bone in the head of an embryo 

 cat. The mesenchyme cells are concentrated in the peripheral region to be occupied 

 by the periosteum. A number of cells are imprisoned in an acidophilic fibrous 

 matrix in which calcium salts are depositing. In the 3 small central spaces primary 

 marrow cavities are forming and contain blood elements and osteoblasts. 



into a fibrous membrane, the periosteum. From its inner surface 

 osteoblasts are differentiated and through their activity parallel 

 plates, or lamellte, of compact bone are formed, a process known as 

 periosteal ossification. Blood vessels forming in the connective 

 tissue between the spicules of bone connect with ^'essels develo]iing 

 in the periosteum, and other mesenchyme cells in these regions 

 among the spicules give rise to reticular tissue, adipose cells, and 

 developing blood cells, which together represent the embryonic 

 bone-marrow. 



As a result of ossification to this ])()int, u])])cr and lower, or inner 

 and outer, plates of compact bone are connected by a central region 

 of spongy bone. This early bone is not j^ermanent, for a moulding 

 of the mature bone involves resorption of nuich of the first bone 



