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Mysteries of leg development (cf. text and Fig. 5.1e).
a. Quadrant Regeneration Mystery. Unlike the wing disc, where every quadrant duplicates, the upper medial (UM) quadrant of the leg disc regenerates (black triangles; cf. key in Fig. 4.5). UM regeneration is seen either with  and without  the endknob (center region).
b. Dorsal Remnant Mystery. When cells are killed by X-rays  or cell-lethal mutations [2102, 3705], D structures tend to remain, while V ones are lost (except near branch tips). First-leg discs, which are conjoined , fuse medially instead of duplicating, and losses are symmetrical on left (L) vs. right (R). In this contour map, darker zones denote higher frequencies (0-30; -60; -90; -100%) of elements present in 65 pairs of fused 1st legs.
c. Triplications Mystery. Triplicated legs possess a 'stem'  and a symmetrical outgrowth. V-side outgrowths tend to diverge, while D-side ones converge, and the (2/6.5) line between cohorts is sharp.
Maps in a, b, and c are redrawn from , , and  respectively.
N.B.: Medial and lateral mean toward or away from the larval midline, while A, P, D, and V refer to prospective or actual directions in the adult.