During a gross anatomy lecture, a professor presents the following observation: in a neurologically intact adult, the patellar reflex (spinal levels L2–L4) is elicited by tapping the anterior knee, whereas the Achilles reflex (spinal level S1) is elicited by tapping the posterior ankle. Although L2–L4 are numerically higher cord segments than S1, their dermatomal territories occupy the anterior lower limb while S1 occupies the posterior surface — the opposite of a simple craniocaudal gradient. Both reflexes are intact and the limb is structurally normal.
Which of the following embryologic events best explains why higher lumbar spinal levels (L2–L4) map to the anterior lower limb while the lower sacral level (S1) maps to the posterior surface?