During their development, the limbs undergo several changes of position. In the very early development they point in a caudal direction, bat soon project outward (ie, laterally) almost at right angles to the body wall. At this stage the flexor surface of each limb is ventral and the extensor surface is dorsal. The preaxial border of each limb lies cranially, whereas the postaxial border lies caudally. Next, the limbs bend in a ventral direction at the elbow and knee. Consequently, the elbow and knee now point outward, while the palms and soles face the trunk. Finally both the sets of limbs undergo a rotation 90 of degree on their longitudinal axes, but in opposite directions. The upper limbs rotate laterally, so that elbows come to face posteriorly and the extensor muscles come to lie on the lateral and posterior aspects of the limb. The lower limbs rotate medially through 90 degrees, so that the knees face ventrally and the extensor muscles come to lie on the anterior aspect of each lower limb.