The mesenchyme cells that will form the heart originate in the epiblast layer of the embryo, migrate through the primitive streak into mesoderm, and from their positions in the splanchnic layer, fold medially, ventral to the foregut, to form the cardiac tube. This tube shortly loops to the right and begins its morphogenetic differentiation into chambers, septa and valves. Previous mapping experiments in the chick embryo have resulted in a fairly concise account of the earliest periods of cardiogenesis. However, scant information is available about the intermediate and final positions of the cardiogenic cells and the way they differentiate into the various components of the definitive heart. By utilizing a variety of marking and chick embryo culture methods, the proposed research will trace these individual heart components (chambers, septa and valves) to their origins. This information will not only lead to better comprehension of the cardiac anatomy of normal hearts and that found in congenital heart disease, but may result in more rational methods of surgical management of these disorders. Some of the results should prove useful in understanding the etiology of the arrhythmias associated with the anomalous atrioventricular connections of the Wolff-Parkinson-White syndrome and other related conditions.