BONE 



287 



In hyaline cartilage, the matrix remains hyaline. In fibro-carlilage, 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. 



Periosteum 



Osteoblast 



Bone matrix] 



Bone matrix 



Bone cell 



/'/' ' 



>,. Hill 



Bone 



Osteoclast 

 Fibrilla 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 

 lamella 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, 



