46 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



parently of very slender fibers decussating obliquely, but coalescing at the 

 points of intersection, as if here the fibers were fused rather than woven 

 together. The reticular lamellae are perforated by the perforating fibers of 

 Sharpey, which bolt the neighboring lamellae together, and may be drawn 

 out when the latter are torn asunder, figure 56. These perforating fibers 

 originate from ingrowing processes of the periosteum, and in the adult still 

 retain their connection with it. 



FIG. 56. Lamellae Torn off from a Decalcified Human Parietal Bone at some Depth 

 from the Surface, a, a, Lamellae, showing reticular fibers; b, b, darker part, where several 

 lamellae are superposed; c, perforating fibers. Apertures through which perforating fibers 

 had passed, are seen especially in the lower part, a, a. of the figure. (Allen Thomson.) 



Development of Bone. From the point of view of their development 

 all bones may be subdivided into two classes: 



Those which are ossified directly in membrane or fibrous tissue, e.g., the 

 bones forming the vault of the skull, parietal, frontal, and a certain portion 

 of the occipital bones. 



Those whose form, previous to ossification, is laid down in hyaline carti- 

 lage, e.g., humerus, femur, etc. 



The process of development, pure and simple, may be best studied in 

 bones which are not preceded by cartilage, i.e., membrane-formed. Without 

 a knowledge of ossification in membrane it is difficult to understand the much 

 more complex series of changes through which such a structure as the carti- 

 laginous femur of the fetus passes in its transformation into the bony femur 

 of the adult (ossification in cartilage}. 



Ossification in Membrane. The membrane, afterward forming the 

 periosteum, from which such a bone as the parietal is developed, consists 

 of two layers, an external fibrous and an internal cellular or osteogenetic. 



