46 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



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. 



The external layer consists of ordinary connective tissue, with branched 

 corpuscles here and there between the bundles of fibers. The internal layer 

 consists of a network of fine fibrils with nucleated cells and ground or cement 

 substance between the fibrous bundles. It is more richly supplied with 

 capillaries than the outer layer. The relatively large number of its cellular 

 elements, together with the abundance of blood-vessels, clearly mark it as 

 the portion of the periosteum which is immediately concerned in the for- 

 mation of bone. 



In such a bone as the parietal there is first an increase in vascularity, 

 followed by the deposition of bony matter in radiating spicula, starting 

 from a center of ossification. These primary bony spicula are osteogenetic 

 fibers, composed of osteogen, in which calcareous granules are deposited. 

 Calcareous granules are deposited also in the interfibrillar matrix. By 

 the junction of the osteogenetic fibers and their resulting bony spicula a 

 meshwork of bone is formed. The osteoblasts, being in part retained within 

 the bone trabeculae thus produced, form bone-corpuscles. Lime salts are 

 deposited in the circumferential part of each osteoblast, and thus a ring 

 of osteoblasts gives rise to a ring of bone with the remaining uncalcified 

 portions of the osteoblasts embedded in it as bone-corpuscles. At the same 

 time the plate increases at the periphery by the extension of the bony spicula 

 and by deposits taking place from the osteogenetic layer of the periosteum. 

 The bulk of the primitive spongy bone is gradually converted into compact 

 bony tissue of the Haversian systems. 



Ossification in Cartilage. Under this heading, taking the femur 

 as a typical example, we may consider the process by which the solid cartilag- 

 inous rod which represents the bone in the fetus is converted into the hollow 

 cylinder of compact bone with expanded ends formed of cancellous tissue 

 in the adult long bone. 



The fetal cartilage is sheathed in a membrane termed the perichondrium, 

 which resembles the periosteum described above. Thus, the differences 

 between the fetal perichondrium and the periosteum of the adult are such 

 as usually exist between the embryonic and mature forms of connective tissue. 



There are several steps in the transformation of the fetal cartilage to the 

 adult bone, due to the fact that there is first an impregnation of the cartilage 

 with lime salts, followed later by the resorption of this entire material with 

 formation of the embryonic spongy bone, which is later replaced by the per- 

 manent bone. The complicated phenomenon takes place in steps or stages as 

 follows: 



