ossification
Early in the development of a human fetus, the skeleton is made entirely of cartilage . The relatively soft cartilage gradually turns into hard bone through ossification . This is a process in which mineral deposits replace cartilage. As shown in Figure below , ossification of long bones, which are found in the arms and legs, begins at the center of the bones and continues toward the ends. By birth, several areas of cartilage remain in the skeleton, including growth plates at the ends of the long bones. This cartilage grows as the long bones grow, so the bones can keep increasing in length during childhood.