Proposed embryological positions of digit progenitors and subsequent biological/morphological digit identities. (a-d) The embryological positions of the digit progenitors are represented by arabic numbers 1 to 5. Roman numerals I to V designate the proposed biological/morphological digit identities that ultimately arise from the progenitors of each embryological position. The zone of polarizing activity is shown in pale orange, and the sonic hedgehog signaling gradient is shown in grey shading. (c) An arrow shows the movement of cells out of the zone of polarizing activity by HH22 in the frame-shift model. Digit nomenclature: Embryonic digit position: 1 to 4 (anterior to posterior) indicate the positions of the digit progenitors in the digit-forming region during their specification; Biological/morphological digit identity: I to IV (anterior to posterior); Experimental digit position assignment: A to D (anterior to posterior).