Diffusion of the small molecule fluorescein into the medaka embryo. Embryos at various stages were incubated with 10 mg/ml fluorescein for 40 minutes at 27°C. Pictures were taken after 30 minutes of washing and the fluorescence intensity was measured. The dotted lines demarcate the outlines of: (A,B) yolk and blastoderm, and (C,D) yolk and embryo. Embryos are shown in lateral view. Black arrow indicates the liver and gall bladder. (A-D) Distribution of fluorescein uptake during four representative embryonic stages: A, stage 2a; B, stage 8; C, stage 26; D, stage 38. The internal concentration of fluorescein (average distribution in embryo/yolk assumed) for different embryonic stages is shown in (E). Scale bar 250 μM. Abbreviations: hpf, hours post fertilization.