In reptilles (varanids, crocodilians) we study correlation between the degree of fibrotization of the atrioventricular canal, ventricular septation and their activation system, and conduction system morphology. Similar studies are conducted in mouse and chick embryonic models of deficient ventricular septation. We expect the results to provide new information about the phylogenesis of the cardiac conduction system, which is impossible to obtain by any other means.