Photocytes in control embryos and cleavage-arrested embryos. (A, A', A'') In normal embryos, photocytes (white arrows) are visible at 9 to 10 hpf in the endoderm beneath the comb rows. In this live animal, viewed from the aboral pole, (A) shows nuclei in blue (Hoechst 33342 staining) overlaid with photoprotein fluorescence (green). (A') Photoprotein fluorescence as viewed under the GFP channel. (A'') DIC image of the same embryo shown in (A) and (A'). (B) Photoprotein fluorescence of a control animal. (C, D, E) E macromeres were labeled with DiI at the 8-cell stage and embryos were cleavage arrested at the 16-cell stage. Photoprotein is visible as green dots (arrowheads), in cells that are not labeled. (F) Photoprotein in an embryo that was cleavage arrested at the 16-cell stage. hpf, hours post fertilization. Additional files8 and9 show videos of the light-emitting photocytes of the specimens shown in figures B and F.