Foetal bone formation. Coloured scanning electron micrograph (SEM) of a section through a foetal long bone showing bone formation (ossification). An area of rough-looking bone can be seen at upper right. Bone is forming at the epiphyseal plate, the boundary between the bone and the smooth cartilage below and to its left. In this endochondral ossification, the cartilage, from which foetal bones are initially formed, is gradually replaced by bone. Bone formation starts in the centre of the bone shaft (diaphysis) and progresses towards the cartilaginous extremities (epiphyses). Some cartilage is still present in childrens' bones. Magnification unknown.