Examination of living embryos has shown that the heart is a functionally active organ from a very early stage of its development. At all periods of life the result of the functional activity is in essentials the same, viz. the propulsion of the blood in a definite direction through the heart into the vessels arising from it; but the mechanism for effecting this propulsion undergoes profound alterations, and the heart becomes transformed from a simple continuous tube, destitute of valves, whose walls contract in a rhythmic peristaltic wave, into a complex four-chambered organ, divided into right and left portions, which are ultimately completely separated from one another, possessing valves, and contracting not in a peristaltic wave but in alternating consecutive contractions of the atria and ventricles of the right and left sides simultaneously. Coincidently with the changes in the heart itself, profound alterations occur in the vessels leading to and from the heart. In this combination of simultaneous development and functional activity the heart differs from the other organs of the body, and hence its development presents special problems involving the function as well as the structure of the different parts. Our knowledge of the development of the heart in man cannot yet be said to be complete.