Chap. VII.] 



BONE. 



61 



74. I nt or membranous formation. All 



bones not preformed in the embryo as cartilage are 

 developed directly from the periosteum in the manner 

 of the periosteal bone just described (Fig. 37). Here also 



a 



Fig. 37. A small mass of Bone Substance in the Periosteum of the 

 Lower Jaw of a Human Foetus. 



a, Osteogenetic layer of periosteum ; 6, multinucleated giant cells, myeloplaxes. 

 The one in the middle of the upper margin corresponds to an osteoclast, 

 whereas the smaller one at the left upper corner appears concerned in the 

 formation of bone. Above c the osteoblast cells become surrounded by 

 osseous substance and thus become converted into bone-cells. (Atlas.) 



the new bone is at first spongy bone, which in its inner 

 layers gradually becomes converted into compact bone. 



In all instances during embryo life and after 

 birth the growth of a bone in thickness takes place 

 after the manner of periosteal bone ; this is at first 

 spongy, but is gradually converted into compact bone. 



75. All osseous substance is formed in the embryo 

 and after birth by the osteoblast or marrow cells 

 (Gegenbaur, Waldeyer) : each osteoblast giving origin 

 to a zone of osseous matrix, and in the centre of this 



