Electrical records of the heart of the chick embryo can be obtained at 40 hours (15 somites). These earliest records show a fast and a slow component making up an R-T complex. The electrocardiogram of the third-day embryo is practically identical with that of the adult. The development of fibrils and of syncytial-like structure have been shown, however (Wieman, 1907), to appear first at the fifth day and therefore the characteristics of the electrical record of the chick's heart cannot depend on the presence of these structures.
The time relations of the earliest electrical records show a remarkable resemblance to those found in the adult hen although the adult heart weighs 14 gm. and the early embryonic heart not more than 1 mg.