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Scanning electromicrographs of human forelimb buds on gestational day 29. A: Transverse section through a limb bud. From Kelley (1985). B: External view at a similar stage of development. AER, apical ectodermal ridge; LM, limb mesenchyme; PZ, progress zone; DE, dorsal ectoderm; VE, ventral ectoderm; ZPA, zone of polarizing activity. From Larsen (2001).
