Practice question

Timing of AER removal reveals progressive proximal-distal specification and irreversible commitment of progenitors. Early fate mapping shows proximal progenitors exit progress zone first under high retinoic acid and Meis activity, distal progenitors remain labile longest. If AER excised at Hamburger-Hamilton stage 18-19, before zeugopod and autopod progenitors expand under FGF influence, only humerus specified during high RA period remains with absence of radius ulna and digits. Later removal preserves humerus plus radius ulna but lacks digits. Very late removal yields almost complete limb with minor distal loss. Classic Saunders experiments established early removal yields humerus only.