Fig. 2. Migrations of the DA and DB motoneuron axons from the ventral nerve cord. A, Schematic transverse section of the adult hermaphrodite body wall. The different longitudinal nerves (circles) are located between the basal surface of the epidermis and the basement membranes. Muscle cells project arms to the ventral and dorsal nerve cords to form neuromuscular junctions. The extent of the dorsal migrations of DA and DB motoneuron axons was measured by scoring the number of axons that cross each of three dorsoventral positions: VSL, L, and DSL. In wild-type embryos, processes from DA and DB motoneurons migrate circumferentially from their cell bodies, across all three positions, to the dorsal midline. Each neuron is bipolar, and the second process migrates longitudinally in the ventral nerve cord. B,unc-6, unc-5, and unc-40 are required to guide the axons to the dorsal midline. In the mutants, processes leave the ventral nerve cord and cross the VSL position, but the axons wander and often join longitudinal nerves before reaching the DSL position. Axons also tend to wander in unc-6(-) animals that express the unc-6ΔC transgene, but the same number of axons as in wild-type animals eventually cross the L and DSL positions to reach the dorsal midline, and a dorsal nerve cord is formed. Also, additional processes leave the ventral nerve cord and cross the VSL position. These axons fail to migrate across the lateral epidermis to position L. In unc-5; unc-6 andunc-40; unc-6 mutants that expressunc-6ΔC, the number of axons that leave the nerve cord and cross the VSL position is near normal, but fewer axons reach the DSL position because the axons wander. Axons were scored on both sides of the animal, but only circumferential migrations on the left side, between the retrovesicular ganglion (near the terminal bulb of the pharynx) and the vulva, are reported here (DA1, DB2, DA3, DB4, DA4, DB5 motoneuron axons). Data points are means ± SEM (n = 50).