Completion of the epithelial to mesenchymal transition in zebrafish mesoderm requires Spadetail

► We visualize mesodermal cells undergoing the epithelial-mesenchymal transition. ► We show that cells undergoing the transition are in a highly blebbing state. ► We show that spadetail mutant mesoderm is trapped in the transition state. ► We describe a novel system for examining transition state cells. ► We show that transition state cells are more adhesive than mature mesoderm.