Fig. 2. Temperature-sensitive period of hdl mutant producing headless pharate adults. Embryos derived from a hdl/ci^D stock were collected for 4 hours and raised at 19.5°C (A), or collected for 2 hours and raised at 28°C (B), until the temperature was shifted to 28°C and 19.5°C, respectively, during embryonic development at the times indicated on the abscissa. The fraction of headless phenotypes among pharate adults is plotted against the time corresponding to the average age of the embryos at the time of the temperature shift. Points on the ordinate in A and B represent the fractions of headless pharates observed when the temperature was kept constant at 28°C (A) or 19.5°C (B) throughout development. Note that in both shift-up and shift-down experiments the temperature-sensitive period ends at stage 16 and begins at stage 12, the time when toy and ey transcripts begin to appear in the anlagen of the eye-antennal discs and in the optic lobe (Czerny et al., 1999). Embryonic stages are numbered according to Campos-Ortega and Hartenstein (Campos-Ortega and Hartenstein, 1997).