Representative embryonic mutant phenotypes of axonal projections in the ventral nerve cord. Images of ventral nerve cords in horizontal view (dorsal up) of embryos that are wild type, mutant or display targeted expression of genes in subsets of neurons. Genotypes are indicated in black at the top, antibody stainings in grey (abbreviations: Robo, Roundabout; Comm, Commissureless; Fas2, Fasciclin 2; Drl, Derailed; Rep, reporter gene; eg, eagle; ftz, fushi tarazu; ap, apterous; unc5, uncoordinated 5; NCad, N-Cadherin; gcm, glia cells missing; nrg, neuroglian; nrt, neurotactin; "X ÷ Y", expression of gene Y driven by the promoter of gene X). BP102 antiserum labels the complete neuropile (i), consisting of two connectives (asterisks) and, per segment, an anterior (white arrow) and posterior (black arrow) commissure. At stage 16 Fasciclin 2 labels three prominent longitudinal fascicles per connective (ix). (ii-viii) Expression patterns of genes involved in midline crossing and, below (x-xvi), respective loss-of-function phenotypes (see text for details). (xvii-xxii) Regulation of growth through the anterior versus posterior commissure, with loss of posterior (xvii) or anterior (xviii) commissure, specific expression of Derailed (xix) in anterior and its ligand Wnt (xx) in posterior commissure, and shift of posterior commissure neurons (eagle-Gal4; xxi) to the anterior commissure (white arrow head in xxii) upon Derailed expression in their axons. (xxiii) Same eagle-Gal4 neurons partially lack commissural projections in netrin A+B mutant background (compare white arrowhead in (xv)). (xxv-xxxi) Detailed phenotypic studies using identified apterous-Gal4 neurons, which project transversely to the medial connectives where they form a longitudinal fascicle (xxv); as indicated by white arrows, they stall prematurely upon Derailed expression (xxvi), project across the midline upon Comm expression (xxvii), collapse towards the midline in robo mutant background (compare x), shift to lateral positions upon Robo2+3 expression (xxix), project out of the CNS upon Unc5 expression (xxx), or turn prematurely from transverse into longitudinal direction in N-cadherin mutant background (xxxi). (xxiv, xxxii). Connectives are affected upon ablation of longitudinal pioneer neurons (xxiv) or longitudinal glia cells (xxxii). (xxxiii-xl) Various neuropile phenotypes in embryos mutant for transmembrane molecules (xxxiii-xxxv) or factors involved in cytoskeletal regulation (xxxvi-xl). Images were taken, with permission, from  (i, xxxv),  (ii-iv, xii, xxix),  (v, top),  (v, bottom),  (vi),  (vii, xv),  (viii),  (ix, xxxiii),  (x, xi, xiv),  (xiii, xvii),  (xvi, xxx),  (xviii),  (xix, xxi, xxii, xxv-xxvii),  (xx),  (xxiii),  (xxiv),  (xxviii),  (xxxi, xxxiv),  (xxxii),  (xxxvi-xxxviii),  (xxxix, xxxx). Images were modified to grayscale and adapted to size.