Medaka gastrulation—is an essential process for medaka morphogenesis. During gastrulation, various modes of cell movements take place coordinately. (A) At the beginning of the gastrulation (the shield stage), the blastoderm extends and thins simultaneously to cover the yolk cell (epiboly, red). At the boundary of epiboly, some internalized cells form the germ ring (internalization, blue). On the dorsal side, mesendodermal cells involute (involution, yellow). (B) 65% epiboly; the blastoderm covers approximately 65% of the yolk cell. The lateral cell group migrates dorsally while stretching along the animal–vegetal axis (convergence, green). On the dorsal side, mesendodermal cells migrate anteriorly, and the elongation of embryonic body is observed along the anterior–posterior axis (extension, white). (C) At the YPC (yolk plug closure), the end of the epiboly, the anlage of the embryonic body is observed, which has the embryonic body axes.