immediately_preceded_by developmental stage Medaka stage 16 Medaka stage 17 Head formation OlatDv:0000200 Zoological Science 11: 825-839 (1994) - 1 day 1 hr Stage 17 (1 day 1 hr) Early neurula stage (Head formation): The yolk sphere is nearly covered by the thin blastoderm leaving a small area around the vegetal pole (yolk plug) ex- posed. The head (rudimentary brain) is recognized anteriorly in the distinct embryonic body. A beak-like mass of cells is seen in front of the head. A few small vacuoles (Kupffer's vesicles) appear at the underside of the caudal (posterior) end of the body, which is in contact with a small blastopore. - 1 day 1 hr early neurula stage medaka_ontology