In the course of the growth of the foetus much of the skeleton is laid clown in a soft flexible substance termed cartilage or gristle, out of which bone is ultimately developed by a succession of changes, including the deposition of mineral matter into its structure. In long bones this process of ossification is first commenced in the centre of the diaphysis or shaft, from which it spreads to the extremities, where it is ultimately met by an ossifying centre from each. The two ends are termed epiphyses, and during the period when the animal is growing they may be, and sometimes are, broken away from the shaft by muscular contraction and other forms of violence. Where a considerable projection appears on a bone, as on the upper end of the femur, they are produced from separate centres of ossification and known as apophyses.
Growth in length takes place between the ossifying centre in the shaft and those of the extremities; in thickness it proceeds from the inner surface of the periosteum, which lays down bony matter layer upon layer.
Flat bones, such as those which enter into the formation of the cranium, the scapula or blade bone, etc., are developed between two membranes and not, as in long bones, from a pre-existing model of cartilage. The former is termed intra-membranous ossification, the latter intra-cartilaginous.