The arm and leg primordia are initially similar. With respect to a cranio-caudal plane (plane passing through the proximo-distal and cranio-caudal axes of the limbs) the extensors develop on the dorsal side and the flexors on the ventral side. The thumb and the big toe are both initially oriented in a cranial direction. The elbow and the knee develop at the bends between the upper arm and forearm, and the upper and lower leg respectively. They are initially oriented laterally. In the adult (in normal position), the thumb is orientated laterally and the big toe medially. There is then a total rotation of about 90 degrees outwards for the arm, and about 90 degrees inwards for the leg. Due to this, the extensors lie dorsally on the arm and ventrally on the leg (animation: growth movements of the limbs).