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Fig. 1 CaP morphology is extremely reproducible. (A) The schematics illustrate the stereotypical morphology and axonal trajectories of the RoP (red), MiP (green) and CaP (blue) primary motor neurons at ~48 hpf from lateral (left) –and cross-sectional (right) perspectives (Myers et al., 1986). Dashed horizontal lines correspond to the horizontal myoseptum (choice point). Left: chevrons show the hemisegment boundaries. Anterior is to the left and dorsal is up. Right: Dorsal is up. (B) Three adjacent hemisegments from a germline transgenic animal [Tg(Mmu.MNX1-MNE:EGFP)] that expresses EGFP in motor neurons were imaged at ~48 hpf. Arrows indicate the characteristic loop of the CaP axon under and around the ventral muscle. In this and subsequent figures, confocal images were projected and lateral or quasi-lateral views are shown. Anterior is to the left and dorsal is up. (C) Two adjacent hemisegments from an animal that was injected with the EGFP plasmid for transient expression in motor neurons were imaged at ~48 hpf. Arrows indicate the characteristic loop of the CaP axon under and around the ventral muscle. In this and subsequent figures, the dashed line indicates the location of the horizontal myoseptum (choice point). (D) Projected confocal images were acquired from an animal transiently expressing EGFP in all three primary motor neurons in one hemisegment at ~36 hpf (left) and ~48 hpf (right). (E) Using the confocal image stack acquired at ~48 hpf, each neuron was traced using Neurolucida. Projected traces are shown from lateral (left) and cross-sectional (right) perspectives. In D and E, and in subsequent figures unless otherwise noted, arrowheads indicate cell bodies and arrows indicate axons: red, RoP; green, MiP; blue, CaP. In E and in subsequent Neurolucida traces, circles indicate branch points.