[Development of the avian and mammalian heart prior to the onset of blood circulation, studied in chick and mouse embryos].

Heart development, its morphological and functional development prior the start of blood circulation were compared in chicken and mouse embryos. At beginning of neurula stage, when also somites appear, the cardiogenic mesoderm forms into tissue of epithelial type and cardiac plate is developed in wall of pleuroperitoneal cavity. In conjugated cardiac plate, primordial cardiac cavity fundaments appear in the about 30-hour old chicken embryo with 6-7 somites. First myoblasts (and spontaneous action potentials) can be observed in the ventricular part of embryos with 7 somites. Hence, muscular differentiation is expanded to atrial section. In embryo with 9-10 somites straight cardiac tube is developed, its ventricular section pulsates feebly, but it is unable for blood circulation yet. Pacemaker tissue of cardiac tube is in the atrium. At the end of second day the first curvature of heart appears and myocytes of venous sinus become pacemaker. At the end of neurula stage the blood circulation starts in two-day old embryo with 16 somites. Development of heart of mammalian embryos differs in several aspects from cardiac development of avian ambryos. In mammals transformation and differentiation of cells in cardiac plate and primordial cardiac fundaments have a cranio-caudal trend and in nearly same developmental stage as in chicken. In mouse circulation is started by curved cardiac tube of embryo with 10-12 somites (eight and half day old) at the end of neurala stage.