immediately_preceded_by developmental stage Medaka stage 11 Medaka stage 12 OlatDv:0000150 Zoological Science 11: 825-839 (1994) - 10 hrs 20 min Stage 12 (10 hrs 20 min) Pre-early gastrula stage: The blastoderm has flattened down onto the yolk sphere so that its outer surface follows the curvature of the yolk sphere. The cell layers are slightly thicker on one side. The diameter of the cells in the central region of the blastoderm remains about 20 m. - 10 hrs 20 min medaka_ontology pre-early gastrula stage