Summary of heart tube lumen formation in wild-type and mutant embryos. Lumen formation in the D. melanogaster heart depends on specific sites of adhesion and de-adhesion between contralateral CBs. We propose that Slit/Robo signaling is required to maintain de-adhesion along the central apical domain of contralateral CBs. In wild-type embryos, CBs are specifically adhered at dorsal and ventral attachment points where E-Cad accumulates. Between these points, the apical membranes of the CBs are repelled from each other, allowing for the formation of a lumen. In robo LOF or shg/E-Cad GOF embryos, contralateral CBs remain adhered to each other, resulting in a block in lumen formation and an apical accumulation of E-Cad.