GROWTH 



349 



each individual bone increases its dimensions by the addition of 

 material along the sutural surfaces. In Reptiles the sutures 

 tend to remain open throughout life. 



735. In the ossification of the long bones like the femur, the 

 bone is first deposited on the surface of the cartilage in its 



B 



Fig. 217. — Cross section of a vertebra of an embryo (pig) showing centres of 

 ossification. The parts in black are cartilage. At three points the cartilage is 

 being replaced by bone; in the centrum (A) and in the two sides of the neural 

 arch (B). As the bony parts grow outward into the cartilage the cartilage 

 between them also grows. Thus the vertebra increases in size with the growth 

 of the body. Finally, however, all the cartilage is replaced by bone and the 

 parts unite to form a single body of bone. 



middle region. The two ends remain cartilaginous for some time 

 and grow by the growth of the cartilage. In the middle region 

 or shaft the growth of bone continues by the addition of new 

 layers to the outside, and the new layers extend gradually 



