Practice question

Planarian tissue retains positional memory via muscle cells expressing position control genes that persist after amputation. Middle third fragment lacks both tips but keeps graded Wnt activity; anterior wound upregulates notum via Erk signaling, posterior wound upregulates wnt1, rebuilding opposing poles. Resulting blastemas acquire distinct profiles guided by underlying muscle map. Anterior blastema generates brain, eyes, and pharynx extension, posterior generates tail with copulatory apparatus. Outcome shows middle fragment forms both head and tail because internal positional information rapidly re-creates polarized signaling centers after injury.