MEMBRANOUS BONE-FOEMATIOy. 



59 



FIG. 26. 



these walls are only partially absorbed. The larger spaces 

 thus formed are termed secondary areolce. Connective-tissue 

 cells now deposit themselves on the calcified walls of the 

 secondary areolse and become osteoblasts, or bone-formers. 

 They secrete a layer of bony - 

 substance, in which they and 

 their processes become buried ; 

 they thus become bone-cells, 

 in lacuna?, with their proc- 

 esses in canal iculi. When 

 one layer of bone is thus de- 

 posited, other osteoblasts be- 

 come fixed on the new sur- 

 face and form another layer. 

 This process proceeds from 

 the periphery of the spaces 

 inward, until only a small 

 passage is left, the Haversian 

 canal, the surrounding lamellae 

 constituting a Haversian sys- 

 tem. The formation of bone 

 proceeds in this manner away 

 from the centres of ossification; 

 hence there are layers of grow- 

 ing cartilage intervening be- 

 tween the different ossifying 

 areas, as between the shaft 

 and the epiphyses of long 

 bones. .Not until the ^ bone Development of bone cartilaginous 



attains its lull growth are method ( Klein). 



these intervening cartilagi- 

 nous portions fully converted into bone. The bone formed 

 by the cartilaginous method is temporary, being subsequently 

 removed and renewed by the membranous method ; while 

 the interior of long bones becomes excavated to form the 

 marrow cavities. 



Membranous bone-formation : Some of the bones, as the flat 

 bones of the skull, are preceded in the embryo by white 

 fibrous membranes. The conversion of these into bony tissue 

 (intramembranous bone-formation) and the formation of bone 



