AbstractThe contribution of the inferior endocardial cushion of the atrioventricular canal to cardiac septation and to the development of the atrioventricular valves was studied in the chick embryo by in vivo labelling techniques. The study was performed in White Leghorn chick embryos in which the dorsal cushion was labelled at stage 20‐21 (Hamburger and Hamilton, 1951), when the endocardial cushions were not yet fused. The embryos were sacrificed at stage 35 (mature heart).These experiments allow us to conclude that the inferior atrioventricular cushion gives origin to: (a) that part of the cardiac septum between the septal insertion of the antero‐septal leaflet of the mitral valve and the fibrous ridge which is the equivalent to the human septal leaflet of the tricuspid valve (atrioventricular septum); (b) the region of the interatrial and the interventricular septa adjacent to the atrioventricular septum; (c) the portion of the antero‐septal leaflet of the mitral valve which inserts into the septum; (d) the fibrous ridge corresponding to the septal leaflet of the tricuspid valve.Microdissection shows that, when they appear at stage 18, the superior and inferior endocardial cushions of the atrioventricular canal are in continuity, without boundaries, with both the interatrial and interventricular septa. Therefore, each atrioventricular orifice opens into its corresponding ventricle, there being no stage in the development of the chick embryo heart in which the atrioventricular orifices are connected to the left ventricle at the same time.The development of the atrioventricular canal is similar in the chick and