Schematic overview of heart development in higher vertebrates. Schematic overview of heart development in higher vertebrates. (A) The early heart tube (purple) has a primitive phenotype and this is reflected in a sinusoidal ECG (depicted below). Non-myocardial tissue at the inflow and outflow tract regions of the tube is depicted in yellow. (B) As development proceeds, the myocardium of the chambers (grey) expands from the outer curvatures of the primary heart tube, whereas non-chamber myocardium (purple) of the sinus venosus (SV), atrioventricular canal (AVC), outflow tract and inner curvatures does not expand. The sinoatrial node forms in the SV, whereas the atrioventricular node and atrioventricular junction form within the AVC. The ventricular septum crest part of the primary ring will form the atrioventricular bundle. The bundle branches and peripheral ventricular conduction system, including the Purkinje fibres (not shown), develop from ventricular trabeculations. The trace below depicts an ECG recorded at E9.5. A, atrium; LV, left ventricle; RV, right ventricle. Jan Hendrik van Weerd, and Vincent M. Christoffels Development 2016;143:197-210 © 2016. Published by The Company of Biologists Ltd