The peripheral vasculature starts to develop in the embryo around day 26. A capillary plexus forms dorsal to the aorta as the primitive limb bud develops. Around day 28, this plexus differentiates into the early proximal parts of the upper limb vasculature. By day 33, the subclavian and axillary arteries start to form. At this time, the nerves of the upper limb enter the limb bud. Around day 37, the initial nerve plate divides; the anterior division later gives rise to the musculocutaneous, ulnar, and median nerves, while the posterior division becomes the radial nerve. This time period determines the topography of the branches of the axillary artery relative to the brachial plexus surrounding it. By day 41, the axillary artery has continued distally and become the brachial artery.