AbstractBackground: Ascidians significantly change their body structure through metamorphosis, but the spatio‐temporal cell dynamics in the early metamorphosis stage has not been clarified. A natural Ciona embryo is surrounded by maternally derived non‐self‐test cells before metamorphosis. However, after metamorphosis, the juvenile is surrounded by self‐tunic cells derived from mesenchymal cell lineages. Both test cells and tunic cells are thought to be changed their distributions during metamorphosis, but the precise timing is unknown.Results: Using a metamorphosis induction by mechanical stimulation, we investigated the dynamics of mesenchymal cells during metamorphosis in a precise time course. After the stimulation, two‐round Ca²⁺ transients were observed. Migrating mesenchymal cells came out through the epidermis within 10 min after the second phase. We named this event “cell extravasation.” The cell extravasation occurred at the same time as the backward movement of posterior trunk epidermal cells. Timelapse imaging of transgenic‐line larva revealed that non‐self‐test cells and self‐tunic cells temporarily coexist outside the body until the test cells are eliminated. At the juvenile stage, only extravasated self‐tunic cells remained outside the body.Conclusions: We found that mesenchymal cells extravasated following two‐round Ca²⁺ transients, and distributions of test cells and tunic cells changed in the outer body after tail regression.