Seven-week-old embryo in its amniotic sac, attached to the placenta (right). This embryo measures 18 millimetres. It has all its internal organs and the fingers and toes have become defined. Brain wave patterns can now be recorded and nerves and muscles work together for the first time. The skeletal system, although still cartilage, has developed (ribs seen). The facial features are still flat, but a nose can be detected and a tongue and milk teeth begin to bud. After eight weeks of development the embryo is known as a foetus.