Severe cardiac malformations may involve the atrioventricular valve region, but the sequence of embryonic development of this important area has been little studied. In particular, the basis of atrioventricular muscular discontinuity, except at the conduction system, has remained unexplained. To examine this question, serial histologic sections of human embryos from the Carnegie Embryological Collection and from the Hopkins Pathology Collection were studied and six embryos were reconstructed. The atrioventricular sulcus can be identified in Carnegie stage 10 as an indentation or crease on the right side separating the heart tube from the umbilical vein. By stage 12 the sulcus has deepened and rotated anteriorly as the atria appear and the heart tube elongates rapidly within the confining pericardial space. Selective accumulation of cardiac jelly on the endocardial aspect of the constriction of the heart tube produced by the atrioventricular sulcus is pronounced by stage 14. By stage 16 the separation of the atrioventricular orifice into right and left components is well advanced, and by stage 18 the septation of the atria and ventricles is largely complete. The muscular connection between the atria and the ventricles becomes interrupted around most of the artioventricular sulcus, except for the His bundle, during the latter part of the embryonic period. The topography of the original sulcus assumes a catenoidal or saddle-shaped configuration, i.e., convex in one plane and concave in the perpendicular plane. The tension and pressure relationships in such a structure would favor cardiac jelly accumulation and the eventual disintegration of lines of myocyte connections passing across the groove. The preservation of the His bundle connection is explained by the failure of the sulcus to completely encircle the heart.