Spine + differentiated buds. Two Hox-gene programs: anterior (head) and posterior (limbs).
How it works: The body has a trunk (spine) repeated across segments. Two bud programs grow from branch-tips: Bud A (orange) on the anterior half of segments and Bud B (purple) on the posterior half — like Hox genes expressing different structures along the body axis. budSwitch controls the A/B boundary. trunkGrad tapers the spine across segments (wider head, narrower tail or vice versa). budDensity controls what fraction of branch-tips actually sprout buds — like developmental signal thresholds in real embryology. Not every cell responds to every signal.