Regional Organization of Pre-pacemaker Cells in the Cardiac Primordia of the Early Chick Embryo

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Abstract
There is ample evidence that the cells of the cardiac primordia in the chick embryo have undergone substantial chemodifferentiation long before any histological or morphological sign of the heart appears. Rawles (1943) has localized the position of the potential heart mesoderm in the head-process stage embryo (Stage 5, Hamburger-Hamilton series, 1951) to a pair of lateral heartforming regions. It is at this time that Ebert et al. (1953, 1955) have shown that the same two areas react positively for antigenic combining groups of cardiac actin and myosin. Taking advantage of Spratt's demonstration (1950) of the sensitivity to fluoride ion of embryonic heart, as compared with other embryonic tissues, DufFey & Ebert (1957) have also shown that the cytotoxic effect of this ion on the stage 5 embryo is limited almost entirely to the cells in the heart-forming regions. These regions of precardiac mesoderm can be seen on time-lapse motion picture films as dark condensed areas silhouetted through the endoderm (DeHaan, 1961a, 1963a).