AbstractPatterns of blood flow have been observed at several critical stages in the development of the frog heart. Shortly after the onset of circulation (Shumway stage 20) two inflowing streams were noted to cross in the sinus venosus and then continue through the atrium, parallel to each other. With the development of the hepatic return (stage 24) the right stream becomes much larger and its course diverted into the area that will become the definitive right atrium. The smaller left stream is then noted to flow around the right stream with the development of a spiralling of the streams. The left stream now leads into the area which will become the left atrium. This is the hemodynamic situation as the interatrial septum forms; the septum forms between the streams. Experimental and histological evidence indicate that the interatrial septum is formed by a molding action of the bloodstreams on the tissues of the atrial wall.