The form and position of the developing atrioventricular bundle and its branches has been studied in the hearts of domestic fowl embryos operated upon to induce delayed closure of the interventricular foramen. The operation was performed at 3 days of incubation when the outflow tract of the tubular heart was temporarily distorted by placing a nylon rod beneath the tract for 48 hours. The hearts from operated embryos were sectioned serially and examined with the light microscope at 11 days of incubation; normal hearts of the same incubation age were available for comparison. Small ventricular septal defects were found in a proportion of the operated hearts, and the form and position of developing conducting tissue with relation to the defects was of particular significance. The atrioventricular bundle and the septal component of the ring branch were represented by discrete fasciculi, but the right and left limbs of the bundle were disposed as a drape of tissue overlying the crest of the developing muscular septum, and situated beneath both the ventricular septal defect and cushion tissue of the septal truncus ridge. These observations of the developing conducting tissue in operated hearts are compatible with the concept that the bundle and branches develop normally from part of a precursor tissue distributed as a complete subendocardial sleeve in the tubular heart. They also imply that in the normal adult avian heart the positions of the atrioventricular bundle and its branches denote the junctional zones of the components forming the ventricular septum.