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Fig. 7 Hh activity and heart looping. All are ventral views of MF- 20-stained embryos at 48 hpf. Ventricle is stained darker than aorta. Anterior points upwards. Arrows indicate looping direction. (A-C) Despite morphological abnormalities, the heart of mutants (B,C) always loops to the right, as in wild-type embryos (A). (D-F) Inhibition of Hh activity by cyclopamine during gastrulation does not randomize heart looping. Morphology of the hearts are severely affected by the treatment in both wild-type (D,E) and mutants (F). Occasionally, the hearts of some treated embryos do not loop (E). However, reversal of looping has not been observed.