DEVELOPMENT AND GROWTH OF BONE. 7 



foetus grows its bones increase not only in length, but 

 also in diameter, and this is accomplished by the depo- 

 sition of bone from the under surface of the periosteum, 

 and constitutes another form of ossification known as 

 the subperiosteal. The bone is deposited in layers, 

 forming the secondary lamellae already described. The 

 point at which bone-tissue is first deposited is called the 

 centre of ossification, the earliest evidence of which is a 

 marked vascularity, followed by more or less calcifica- 

 tion. In mtra-cartilaginous ossification a general cellu- 

 lar activity is observed; the cartilage-cells increase in 

 number by a process of fission, elongate, arrange them- 

 selves with their long axes transverse to the long axis of 

 the bone, and in parallel rows. In the matrix, or carti- 

 lage, in which the cells are imbedded, lime-salts are 

 freely deposited, destroying the transparency of the sec- 

 tion, and, if the process of ossification is to be further 

 observed, the section must be treated with an acid to 

 dissolve out the obscuring mineral salts. As they ap- 

 proach the area of ossification the cartilage-cells swell 

 in size, become ovoidal in shape, and their capsules, as 

 well as the surrounding matrix, become infiltrated with 

 lime-salts. They contain a dark, granular matter, called 

 primitive marrow, and are lined with rounded or cuboidal 

 cells. Dipping down from the vessels of the perichon- 

 drium are small blood-vessels, which advance and pene- 

 trate the cartilage. They invade the large cells just 

 described, which, by a process of absorption, begin to 

 communicate, forming canals and spaces lined by layers 

 of rounded cells continuous with the cells beneath the 

 perichondrmm. These cells are of the greatest impor- 

 tance ; some are concerned in the formatien of marrow, 

 while others become the osteoblasts described by Gegen- 

 bauer. 



