Joey is a medaka fish egg that is observed and documented over 8 days as it develops and hatches. On the first day at stage 18, Joey is a small egg that has been alive for 26 hours. By day 2 at stage 23, Joey has a faint heartbeat visible under the microscope after 46 hours. On day 3 at stage 29, Joey's retinas and thick pectoral fins are clearly seen and its heart is visible in photos. Joey continues developing organs and features until hatching on day 8 at stage 36 as a free swimming fish.