<P> When forming intra-atrial septa, atrio - ventricular valves will begin to grow . An intra-muscular ventricular septum begins to grow from the common ventricle to the atrio - ventricular endocardial cushions . The division begins in the common ventricle where a furrow in the outer surface of the heart will appear the interventricular foramen eventually disappears . This closure is achieved by further growth muscular interventricular septum, a contribution of trunk crest - conal tissue and a membranous component . </P> <P> The arterial cone is closed by the infundibular cushions . The trunk cones are closed by the forming of an infundibulotroncal septum, which is made from a straight proximal portion and distal spiral portion . Then, the narrowest portion of the aorta is in the left and dorsal portion . The distal portion of the aorta is pushed forward to the right . The proximal pulmonary artery is right and ventral, and the distal portion of the pulmonary artery is in the left dorsal portion . </P> <P> The rhythmic electrical depolarization waves that trigger myocardial contraction is myogenic, which means that they begin in the heart muscle spontaneously and are then responsible for transmitting signals from cell to cell . Myocytes that were obtained in the primitive heart tube, start beating as they connect together by their walls in a syncytium . Myocytes initiate rhythmic electrical activity, before the fusion of the endocardial tubes . The heartbeat begins in the region of the pacemaker which has a spontaneous depolarization time faster than the rest of myocardium . </P> <P> The primitive ventricle acts as initial pacemaker . But this pacemaker activity is actually made by a group of cells that derive from the sinoatrial right venous sinus . These cells form an ovoid sinoatrial node (SAN), on the left venous valve . After the development of the SAN, the superior endocardial cushions begin to form a pacemaker as known as the atrioventricular node . With the development of the SAN, a band of specialized conducting cells start to form creating the bundle of His that sends a branch to the right ventricle and one to the left ventricle . Most conduction pathways originate from the cardiogenic mesoderm but the sinus node may be derived from the neural crest . </P>

When do the 4 chambers of the heart develop