GROWTH OF BONE. jf 
become enlarged,and thenseparated somelittledistance 
from one another, by the surrounding matrix. Cal- 
careous matter is deposited in this matrix which later 
encloses the cartilage cells. In the meantime the car- 
tilage cells have become much larger and flatter, and 
assumed a definite arrangement, forming a series of 
columns radiating from the centre. While this central 
ossification has been taking place, a growth of osseous 
substance has been formed on the surface of the car- 
tilage, immediately beneath the periosteum. It has 
already been pointed out (p. 8) that, beneath the inner 
layer of the periosteum, there are a series of nucleated 
cells termed osteoblasts; and it is due to these that 
this osseous layer on the outer surface of the cartilage 
is formed. As layer upon layer is formed, lacune 
containing osteoblasts are also formed. After a time 
this outer or osteoblastic tissue becomes irrupted, and 
passes into the inner ossified portion ; ultimately filling 
up the areolz, and finally entirely supplanting the 
ossified cartilage which is gradually absorbed. 
2.—Intermembranous or endochondral ossifi- 
cation. Covering the embryonic cranium, we find a 
tissue or membrane composed of a series of fibres— 
osteogenic fibres—large granular corpuscles and an 
intervening ground substance. Calcium carbonate is 
deposited within these fibres, which form bony spicules ; 
these extend in all directions, in certain regions be- 
coming thicker by the deposition of bony matter, and 
