Slit–Robo functions are required to form the cardiac lumen. Reconstructed time-lapse Z views (6-min intervals) taken from videos (Videos 2–4) illustrating CB shape changes and lumen formation in wild-type (24B-Gal4; UAS actin-GFP; A–L), slit² mutant (A′–L′), and robo/robo2 double mutant embryos (A″–L″) from stages 14–16 of embryogenesis. Mutant alleles are in 24B-Gal4; UAS actin-GFP background to visualize CB shape. Wild-type CBs change their shape from rounded (not depicted), to pearlike (A), to crescentlike (L), whereas the CBs of both mutants (slit² or robo/robo2) remain rounded over the entire period (from A′–L′ to A″–L″). (A, A′, and A″) Red dotted lines delimit the CB shape. Note the highly dynamic actin-GFP accumulation at the sites of cell–cell contact. CBs contact first by the “leading edge” domains (D–D″ and E–E″) where accumulation of actin-GFP (arrowheads) is observed. This cell–cell contact is dramatically enlarged in slit² and robo/robo2 mutants (F–F″ and L–L″, brackets). (L) Red dotted line labels the newly formed lumen, absent in slit² (L′) and robo/robo2 (L″) mutants. Bars, 5 μm.