158 FORMATION OF PRIMARY BONE. 



time, it is removed to give place to another. "While 

 one rod is being formed, some of its neighbours are 

 being removed. Thus the compact tissue is gradually- 

 renovated in every part of its siibstance and without 

 its strength being in the slightest degree impaired, 

 during the time changes, destructive and constructive, 

 are proceeding in its very substance. 



ail" Formation of primary bone. — In the develop- 

 ment of the bones of the skeleton of man and the 

 higher vertebrata, temjyorary cartilage is the seat 

 of formation of a very imperfect and soft spongy kind 

 of osseous tissue, which serves only a temporary pur- 

 pose, and is entirely removed before the more perma- 

 nent form of bone is produced. In the expanded 

 portion of the cranial bones, however, ossification 

 does take place without the previous formation of the 

 temporary or primary bone, as it has been called. 

 The bone in question is not preceded by temporary 

 cartilage, but from the earliest period consists of 

 fibrous tissue, like the periosteum of bones generally. 

 The fibres of this fibrous tissue become calcified, while 

 its masses of bioplasm, or at least some of them, take 

 part in the formation of the future lacunae. 



The more lasting or secondary hone of the skeleton 

 generally is formed at the deep layer of the fibrous pe- 

 riosteum ; and the process agrees very closely in its 

 general characters with that which takes place in the 

 ossification of the expanded portion of the cranial 

 bones. Beneath the periosteum may be seen nu- 

 merous large masses of bioplasm, § 212, for the m.ost 

 part of an oval form, which are the agents' concerned 

 in the production of the formed material of the future 

 bone. And the changes which take place are closely 

 analogous to those which have already been described 

 as occurring in the formation of bone in the frog, 

 § 208. It is, however, much more difficult to study 

 the changes in mammalian bone, in consequence of 

 the firmer consistence of the tissue and the difficulty 



