46 CONNECTIVE TISSUE. 



occipital bone ; while by the intracartilaginous, the humerus, 

 femur, and other long bones are formed. 



Intramembranous Ossification (Fig. 37). This process may be 

 studied in the parietal bone, which, prior to the beginning of 

 ossification, about the seventh or eighth week of fetal life, is a 

 fibrous membrane containing blood-vessels and osteoblasts (Fig. 

 37). The process begins in the center of ossification, which, in the 

 parietal bone, is single, at the parietal eminence. The number of 

 these centers varies in different bones ; in the frontal there are 

 two. 



The embryonic membrane is composed of bundles of fibers, 

 osteogenic fibers, with a granular matrix between them. Both the 



t- 



FIG. 36. Cover-glass preparation from the bone-marrow of dog ; X 1200 (from 

 preparation of H. F. Miiller) (Bohm and Davidoff) : a, mast-cell; 6, lymphocyte; 

 c, eosinophile cell ; d, red blood-cell ; e, erythroblast in process of division ; /, /, nor- 

 moblast ; g, erythroblast. Myelocyte not shown in this figure. 



fibers and the matrix become calcified by the deposition in them 

 of lime salts, and there is produced in them a calcareous mass 

 enclosing blood-vessels and osteoblasts, which latter become bone- 

 corpuscles, and the spaces in which they lie form the lacunae. The 

 blood-vessels permeate the whole, the channels which they form 

 being Haversian canals. It will be observed that in this variety 

 of ossification a membranous structure precedes the bone ; hence 

 the bone is said to Reformed in membrane. 



Intracartilaginous or Endochondral Ossification (Fig. 37). In 

 this form cartilage precedes the bone, and the changes which result 

 in bone-formation take place within it and practically convert it 

 into bone. 



First Stage. In the first stage the cartilage-cells at the center 



