In the 8-, 9-, and 10-day-old mouse embryos, the primitive atria are interconnected with the ventricles via the atrioventricular (A-V) canal. Due to the twisting process of the tubular heart, the wall of the A-V canal establishes continuity not only with the left ventricle but also with the bulbus and truncus arteriosus. At this stage of heart development, the A-V node and bundle have not yet appeared, and, thus, the atrial impulse must be conveyed to the ventricle by the muscle tissue of the wall of the A-V canal, in which two muscle cell layers have been observed. The inner layer extends deep into the left ventricular cavity and is interconnected with both the trabecular system and the ventricular (IV) septum, which begins to develop on the tenth day. In the inner dorsal wall of the A-V canal, the cells are large (∼ 20 μm in diameter) and show a strong PAS reaction. It is likely that these large glycogen-rich cells from which the A-V node primordium develops on the eleventh day play the main role in the A-V impulse conduction. The muscle cells at the ventrolateral walls of the canal are small and form a loose spongy myocardium into which the connective tissue cells begin to penetrate on the tenth day, ultimately to form the annulus fibrosus. At the same time, the outer cell layer of the dorsal wall begins to deteriorate; the cells show vacuolar degeneration, myolysis, and shrinkage necrosis. This process appears to represent a programmed cell death, as was described in the bird heart (Pexieder, 1975). On the basis of morphological data, the sequence of atrioventricular activation before the appearance of the A-V node and bundle is discussed.