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Anatomical and angiocardiographic studies have made possible discussion of hypotheses of the development and differentiation of the conus and of the trunco-conal orientation of the septum. Three autopsy specimens and two angiocardiogrammes of intertwined great vessels with transposition and partial distortion of the great vessels were studied. The specimen of intertwined great vessels comprised a muscular sub-aortic infundibulum posterior to and greater than the pulmonary infundibulum. In the case of transposition the opposite situation was encountered with a muscular pulmonary infundibulum posterior to and greater than the sub-aortic infundibulum; the plane of the aortic valves was higher than that of the pulmonary valves. The specimen of partial distortion of the great vessels comprised a muscular sub-aortic infundibulum posterior to and greater than the pulmonary infundibulum with the aortic valve set higher and in fibrous continuity with the mitral valve. It is concluded that the relations of the great vessels between each other and with the ventricles depend on the orientation of the trunco-conal septum and on the process of incorporation of the cone. The presence or absence of a posterior muscular infundibulum is not related to the growth or differential reabsorption of the cone but to a process of cellular differentiation. There was no relation between the level of the valves and their spatial orientation and the length of the infundibulum. The continuity or discontinuity between the aortic and atrio-ventricular valves is not determined by the level of the aortic valves.