immediately_preceded_by developmental stage Medaka stage 12 Medaka stage 13 OlatDv:0000160 Zoological Science 11: 825-839 (1994) - 13 hrs 0 min Stage 13 (13 hrs 0 min) Early gastrula stage: The blastoderm begins to expand (epiboly, about 1/4 of the yolk sphere) over the surface of the yolk sphere, and the presumptive region of the embryonic shield arises as a thickened margin (dorsal lip) of the blastoderm. It is difficult to recognize the boundaries of the flattened marginal cells. The diameter of the cells in the central region of the blastoderm is 15-20 m. - 13 hrs 0 min early gastrula stage medaka_ontology