Nine salamander hearts have been studied histologically by means of serial sections, cut in each of three planes (transverse, frontal and sagittal), and stained with haemalum and eosin, van Gieson's acid fuchsin and iron-haematoxylin, and by the protargol method of Bodian. This study has demonstrated muscular continuity between the several cardiac chambers, and the entire absence of any specialized muscle or 'nodal tissue' at the junctional sites or in any other part of the heart. The heart muscle forms a continuum. The cardiac muscle fibres are characterized by their large size (i.e. breadth); they have the same general histological characters in all parts of the heart. Measurements are given for the fibres from various parts of the hearts of the salamander and frog. The muscular connexions between the various cardiac chambers have been studied in detail. In each of the chambers the musculature is arranged in a basket-work fashion, but at each of the junctional sites the muscle suddenly changes to a regular circular arrangement. The sinus, at its junction with the right atrium, contains muscle only in its ventral wall, and it is this wall only of the sinus which thus establishes muscular continuity with the ring of muscle (S-A ring) around the sinu-atrial opening. The musculature of both atria is continuous with that of the ventricle in two ways. From the ring of muscle (A-V ring) surrounding the common opening of the atria into the ventricle, the atrio-ventricular funnel dips down into the ventricle, and the caudal border of this funnel is continuous (a) with an invaginated part of the base of the ventricle, and (b) more extensively with ventricular papillary muscles, which, in their turn, are continued into the inner ventricular trabeculae about the middle level of the ventricle. The A-V funnel is homogeneous in structure; no one part of its circumference differs from another. The ventricular muscle is directly continued into that of the bulbus cordis, in which latter chamber the muscle is entirely circular. The course which the wave of contraction takes during its transmission from the sinus throughout the heart has been deduced from the study of the details of the continuity of the musculature of the various cardiac chambers. To a large extent this deduction has been confirmed by superimposing tracings of the outline of the pulsating heart, made from the slow-motion cinephotographic records. This latter study has revealed many of the details of the phases of the cardiac cycle. The delay in the transmission of the wave of contraction from one cardiac chamber to the next is accounted for by the relatively long path which the impulse has to traverse at the junctional sites, where the muscle is arranged in a circular fashion, without postulating the existence of specialized 'block fibres' at these sites. The branching of the muscle fibres has the effect of converting the morphological circular arrangement of the fibres at these junctions into a physiological spiral. The glycogen content of the various parts of the frog's heart, as revealed by staining with carmine, is found to increase in the order sinus, atria, ventricle and bulbus cordis. This is correlated with a similar increasing order of density of musculature and work done, the glycogen being a reserve potency for the energy of muscular contraction. The fact that the intrinsic rhythmic rates of the several chambers decrease in the same order as the glycogen content increases may or may not be coincidental. Cutting and ligature experiments, with cinephotographic and kymographic records, reveal the intrinsic rhythmic rates of the various cardiac chambers of the salamander heart. No satisfactory reason has yet been adduced to account for the different intrinsic rhythmic rates of the several parts of the heart when these are isolated from each other. The dorsal mesocardium has been traced in its entirety. The sinu-ventricular fold is a part of the continuous dorsal mesocardium and does not constitute a direct muscular sinu-ventricular connexion. The distribution of the intracardiac nerve cells has been noted and the probable pathway of migration of these nerve cells in the embryo has been suggested. The significance of the results of this investigation in relation to the phylogeny of the specialized conducting system of the hearts of homoiothermal vertebrates (mammals and birds) is discussed. The view is expressed that the cardiac conducting systems of homoiothermal vertebrates constitute a neomorphic development, correlated with functional requirements, and are not remnants of more extensive tissues of similar structure in the lower vertebrate heart. Variations in this newly evolved formation probably account for the different descriptions of such elements in various mammalian and avian hearts.