Fig. 6. Analogous head defects in bcd NRE and pum^– mutants. (A) Scanning electron microscopy of the embryonic cuticle head (frontal view) and body reference (side view) with maternal genotypes. (Part I) A wild-type head with notable structures (mh, MxSO, AntSO). (Part II) bcd^– null embryos have no head structures. The globular anterior structures resemble posterior ones. (Part III) A wild-type bcd transgene (wt*) rescues the bcd^– null anterior defects. NRE1 (part IV) and NRE2 (part V) mutant bcd transgenes rescue bcd^– anterior development, while additionally inducing defects in some embryos, consistent with the dominant effects in Fig. 5. Protruding structures (arrow). (Part VI) A pum^– (pum¹³/pum¹³) embryo has an analogous medial protrusion and an exposed sclerite resembling those in part V (large and small arrows, respectively). Body side views (right) show abdominal defects documented for pum^– mutation (Barker et al., 1992; Lehmann and Nüsslein-Volhard, 1987; Wang and Lehmann, 1991; Wharton and Struhl, 1991). (B) pum¹/pum¹³ (part I) and pum¹³/pum¹³ (part II) heads (phase contrast) do not develop the mh dorsal projection (large arrow; ventral projection, small arrow). pum¹³/pum¹³ exhibits additional head skeleton defects consistent with its relative allelic strength and its suggested dominant-negative molecular behavior (Barker et al., 1992; Wharton et al., 1998). The protruding lr (black arrow) from defective head involution is in a different focal plane. Black arrows indicate structures residing in a different focal plane. Scale bars: 120 µm in A; 20 µm in B.