Timelapse of pectoral fin development. Live imaging of Tg(α-actin:GFP);Tg(Xla.Tubb:DsRed) larvae to label muscles and axons, respectively. By approximately prim-25 (36 hpf; start of movie), motor axons from nerves 1-3 coalesce at what will form the dorsal plexus and nascent muscle fibers in the pectoral fin bud, located laterally to the axons, have just started expressing α-actin:GFP. Muscle fibers continue to divide and reorganize through the long-pec stage as the fin moves further medial, closer to the plane of the dorsal plexus, and motor axons begin to grow into the abductor and then adductor muscles beginning around the long-pec stage (approximately 13 hours in movie). Concurrently, axons in nerve 4 make a sharp turn dorsally to innervate the fin via the ventral plexus. Thick axon bundles first grow perpendicular to muscle fibers near the proximal fin base, but subsequently axons turn posteriorly to grow mostly parallel to muscle fibers and towards the fin tip. As muscle fibers elongate, branching motor axons follow close behind to form a diffuse innervation network. At the end of the movie (after approximately 37 hours), a simplified innervation pattern is established. Frames are maximum projections through the developing pectoral fin at 30 minute increments. Stills from this timelapse were used in figure 2B. The movie plays at 4 frames per second. Scale bar is 25 microns.