External form

The limb buds firstly grow in a lateral direction. By S14, the fold-shaped anlage gradually comes to occupy an adduction position. This means that the ventral side of the arm bud, in which the flexor muscles arise, comes close to the lateral thorax wall. The distal end of the arm primordium is initially tapered but subsequently widens again considerably at its distal part to form a flat hand plate. At S15, the hand plate can be clearly distinguished from the pedicle-like arm. The arm lengthens and develops into an upper arm and a forearm. The forearm bends against the upper arm. The elbow joint develops at the angle of flexion. The distal part of the forearm rotates inwards. The primordial thumb is initially oriented in a cranial direction before coming to lie more medially through pronation. The elbow turns outwards relative to this movement. In this way, the upper arm is found in a position of abduction and anteversion.