Foetus skeleton. Coloured X-ray of the skeleton of a full-term stillborn foetus. Bone development (ossification) has reached an advanced stage for a foetus. Bone formation begins at the end of the second month of pregnancy, and continues up to adolescence. Here, the early foetal skeleton of cartilage has largely been replaced by bone (pink). The skull, long bones of the limbs, and the spine are well formed. Sutures (joints) in the skull bones are seen. At birth the small bones of the hands and feet, together with epiphyses at the ends of the limb bones, still have to be replaced by bone. These sites of cartilage (spaces) at the joints enable the bones to grow up to adolescence.