Fig. 5. Early expression of ß in the midgut does not rescue the delay in migration caused by the absence of ßPS. ß was expressed earlier than normal in the midgut primordium of embryos lacking ßPS using the 48YGal4 driver, which drives expression in the endoderm and some mesodermally derived tissues (arrow in D). ß levels and endoderm morphology were detected by in situ hybridization against ß transcripts. (A) ß is not normally expressed at stage 9 but visible at stage 12 (C). (B) ß driven by 48YGal4 was detectable from stage 9 onwards. (D) Midgut migration was still delayed when ß was precociously expressed in ßPS^- mutant embryos (compare C and D with wild type embryo in Fig. 4A).