immediately_preceded_by developmental stage Medaka stage 19 Medaka stage 20 4 somite stage OlatDv:0000230 Zoological Science 11: 825-839 (1994) - 1 day 7 hrs 30 min Stage 20 (1 day 7 hrs 30 min) 4 somite stage: A paired placode of otic (auditory) vesicles appears at the posterior region of the head. Depressions begin to form at the dorsal surface of the eye vesicles. Three parts of the brain (the fore-, the mid- and the hind-brain) are discernible. - 1 day 7 hrs 30 min medaka_ontology