Slit expression in CBs rescues Slit localization and lumen formation in how18 mutants. (A–D) Cardiac tubes of stage 16 embryos stained with Dg (green), Slit (red), and How (blue). A′–D′, Slit only; A″–D″, Dg only. (A–D) On the left are dorsal XY views of wild-type (A and C) and how18 mutant (B and D) embryos; and on the right, transverse Z views of the same cardiac tube. (C and D) Slit overexpression under the control of Hand-Gal4, showing a random distribution of Slit in the cytoplasm of the CBs and of the pericardial cells. Slit is also recovered at the L domain of the CBs. (C) In a wild-type background, Slit overexpression does not affect cardiac lumen formation. Although how18 mutant CBs (B) show a strongly reduced lumen and an absence of Slit at the L domain (Fig. 8, I–M), Slit overexpression in this how18 mutant CBs (D) restores lumen formation and Slit localization at the L domain (in addition to its strong expression in the cytoplasm and pericardial cells because of its overexpression). Dashed circles delimit the lumen. (E) A cartoon showing Slit localization and the cardiac lumen in the different conditions depicted in A–D: wild-type (A), how18 mutant (B), Slit overexpression in a wild-type background (C), and Slit overexpression in a how18 mutant background (D). Bars: (all panels except B) 5 μm; (B) 4 μm.