- 2 NORMAL HISTOLOGY. 



the periosteum ; where bone is developed directly from the periosteum, 

 and without being mapped out by primary cartilage, the process is 

 spoken of as intermembranous bone-formation, although differ- 

 ing in no important respect from that producing the periosteal bone. 



Endochondral Bone. The first indications of the future pro- 

 found changes within the solid cartilage correspond in position to 



the so-called centres of ossification, 

 and consist in an increase in the size 

 of the embryonal cartilage-cells, as well 

 as in the amount of intercellular sub- 

 stance separating the cell-nests, followed 

 by a characteristic rearrangement of 

 the enlarged cells into vertical rows 

 or columns ; in the matrix between and 

 around these columnar groups a cal- 

 careous deposit subsequently takes 

 place. These enlarged cartilage-cells, 

 surrounded by the calcified matrix, are 

 the primary areolae of Sharpey. 



cartilage-lacunae and calcified matrix ; c, Simultaneously with the changes 



noted the osteogenetic tissue of the 



periosteum has increased and sent processes from a number of points 

 into the solid cartilage towards ^hereTTtfe 6T ossification ; the progress 

 of the periosteal ingrowth is accompanied by the absorption of the 

 cartilage until the focus of central calcification is reached, when the 

 greatly enlarged cartilage-lacunae are opened up and the spaces 

 brought into direct communication with the primary marrow- 

 cavities. The fate of the cartilage-corpuscles has been the sub- 

 ject of discussion ; it may be assumed as established that these cells 

 undergo degeneration and play no part in the formation of the new 

 bone. This periosteal ingrowth constitutes the vascularization 

 of the cartilage. The process of breaking down the cartilage-cells 

 and opening up the large lacunae goes rapidly forward, resulting in 

 the extension of the primary marrow-cavity; the primary marrow, 

 filling this latter space, is, as already pointed out, the direct deriva- 

 tive of the inner layer of the periosteum. 



The primary marrow-cavity, or medullary space, soon becoming 

 of considerable size, is bordered by the zone of calcifying cartilage ; 

 this area includes the columns of flattened cells and enlarged lacunae, 

 which pass into the broken and partly-absorbed larger lacunae, the 

 secondary areolae, opening into the primary marrow-cavity. 



While the horizontal matrix septa between the transversely ex- 

 panded lacunae disappear, the vertical partitions lying between the 



