The overall structure of the developing heart and the basic arrangement of the definitive heart are preserved across the vertebrates, from fish to man. When first seen during development, the heart is laid down as a linear myocardial tube, with venous and arterial poles (Fig. 17–1A). Conduction through this linear tube begins at the venous pole and proceeds toward the arterial pole. With ongoing development, cavities balloon from the tube, to which additional material is rapidly added at both poles. In the most basic plan, the myocardium that will form the atrial chambers balloons from the primary tube into the dorsal direction, whereas the myocardium of the developing ventricles balloons in ventral fashion (Fig. 17–1B). Concomitant with the ballooning of these pouches from the linear, or primary, tube, it becomes possible to record an electrocardiogram.1 At these early stages, it is not possible to recognize any "conduction tissues" when assessing the cardiac structure histologically. It is possible, however, to distinguish the characteristics of the myocytes in the different parts of the developing heart according to their molecular signatures. Thus, the myocytes of the primary heart tube do not express the connexin proteins responsible for the formation of fast-conducting gap junctions.2 As a consequence, at this early stage, the blood is propelled through the developing heart in a sluggish, peristaltic fashion. In contrast, the myocytes forming the walls of the developing atrial and ventricular chambers express the molecules that ensure rapid conduction and synchronous contraction, such as the gap-junctional proteins connexin 40 and 43.3 This myocardium making up the walls of the developing chambers is called working, or secondary, myocardium. It is possible to distinguish subtypes within the working myocardium.4 The myocytes making up the atrial working myocardium conduct faster and express the rapidly contracting atrial myosin isoform, whereas the myocytes within the developing ventricles express the slow myosin isoform,5 which is also expressed in slow-twitch skeletal muscle. Whereas myocytes within both the atrial and ventricular working myocardial walls express atrial natriuretic factor, the atrial myocytes that are added via the so-called dorsal mesocardium, the area that will eventually form the myocardial sleeves of the pulmonary veins, do not express this gene (Fig. 17–2).