immediately_preceded_by developmental stage Medaka stage 20 Medaka stage 21 6 somite stage Brain and otic vesicle formation OlatDv:0000240 Zoological Science 11: 825-839 (1994) - 1 day 10 hrs Stage 21 (1 day 10 hrs) 6 somite stage (Brain and otic vesicle formation): The optic vesicles differentiate to form the optic cups and the lenses begin to form. The small otic vesicles appear, but they lack otolith. The three regions of the brain are well- defined, and the neural fold (neurocoele) is seen as a median line along the body. The flat body cavity is recognized on the surface of the yolk sphere bilateral to the mid-brain and hind- brain. - 1 day 10 hrs medaka_ontology