Immunohistochemistry was used to detect Snai1 expression in the proepicardium (PE) (arrows) at HH St. 16 (A), and in the epicardial cell layer (Epi) covering the myocardium (arrows) in addition to cells within the sub-epicardial space (arrowheads) at HH St. 31 (B). (C) By HH St. 40, Snai1 expression has decreased but levels are detectable in the epicardium. A, atria; V, ventricle; LV, left ventricle.