BONE 



287 



In hyaline cartilage, the matrix remains hyaline. Infibro-cartilage, the fibrillations of 

 the primitive ectoplasm are converted into white fibers. In elastic cartilage, yellow elastic 

 fibers are formed in the hyaline matrix, according to Mall; before the hyaline matrix is 

 differentiated, according to Spalteholz. Most of the bones of the skeleton are preformed 

 in cartilage. 



BONE 



Bone is a tissue appearing relatively late in the embryo. There are 

 developed two types : the membrane bones of the face and cranium, and 

 the cartilage bones which replace the cartilaginous skeleton. The mode 

 of histogenesis, however, is identical in both. 



'^Jyygilp 



CtoW S , ( : ^~~^%Kffigg^ 



&w& v c s \ e - 

 A * ^v^J^tfs "<^ ft4Bfcu: 



Bone matrix\ 



. 



Bone 



Osteoclast 

 FibrillcB in bone matrix 



FIG. 294. Two stages in the development of bone. A, Section through the frontal bone 

 of a 20 mm. pig embryo (after Mall). X 270. B, Section through the periosteum and bone 

 lamellae of the mandible of a 65 mm. human fetus. X 325. 



Membrane Bone. The flat bones of the face and skull are not pre- 

 formed as cartilage. The form of a membrane bone is determined by the 

 development of a periosteal membrane from the mesenchyma. The bone 

 matrix is differentiated within the periosteum from enlarged columnar 

 cells, the osteoblasts (bone formers). Osteoblasts appear in clusters and 

 from their cytoplasm is differentiated a fibrillated ectoplasmic matrix 

 like that which precedes the formation of connective tissue and cartilage 

 (Fig. 294 A). This fibrillated matrix, apparently by a chemical change, 



