DEVELOPMENT OF BONES. 



269 



The ground substance becomes calcified and the cells become 

 changed into bone cells. In this way there is formed at the 

 border of the cartilage and perichondrium a bony layer, and 

 the perichondrium becomes the periosteum. From the latter, 

 buds grow in toward the areas of ossification, known as peri- 

 osteal buds (Fig. 194). These penetrate the calcified ground 



FIG. 



Hyaline 

 cartilage 



Area of 

 calcification 



.,., Oateogenetic 

 '- - tissue 



Perichondnil 

 bone 



CH Ionics containing 

 "many curtilage cells 



From a longitudinal section of a finger of a three and a half months' human embryo. Two- 

 thirds of the second phalanx is represented. At X a periosteal bud is to be seen. >.">. 



substance of the cartilage, whose cell capsules are broken 

 down, allowing the cells to become free. In this way there 

 gradually arises a cavity in the areas of ossification which 

 forms a primordial or primary medullary cavity, and the first 

 trace of the permanent marrow cavity. In this space are 

 found blood-vessels and cellular elements, which are derived 

 partly from cells brought in by the periosteal buds, and partly 



