288 



HISTOGENESIS 



BONE 



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

 veloped two types, the membrane hones of the face and cranium and the cartilage 

 bones which replace the cartilaginous skeleton. Cartilage bones are not simply 

 cartilage transformed into bone by the deposition of calcium salts, but represent 

 a new tissue which is developed as the cartUage is destroyed. 



Membrane Bone.— The flat bones of the face and skull are not preformed as 

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







Osteoclast 

 FibriUw 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. 



periosteal membrane from the mesenchjona. The bone matrix is differentiated 

 within the periosteum from enlarged cells, the osteoblasts (bone formers). Osteo- 

 blasts 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, by a chemical change apparently, 

 is converted into a homogeneous bone matrix, which first takes the form of 

 spicules. Others view the fibrillated matrix as an intercellular product and the 

 bone matrix as an interfibrillar deposit. However this may be, the spicules 



