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Twenty-two preparations of hearts with preserved atrioventricular canal have been investigated. Primary defect of the interatrial septum is a constant component of this pathological state. The size of the defect defines participation of the atria in formation of the atrioventricular canal. At the same time it is possible to distinguish the following anatomical varieties of abnormal development of the interatrial septum; oblique cleft in the oval fossa, partial absence of the valve in the oval orifice, perforation of the valve in the oval orifice, complete absence of the valve in the oval orifice and certain defects of the interatrial septum situating beyond the oval fossa limits.