The aim of the present study was to describe the fibre architecture of the fetal heart at mid gestation, and to clarify some persistent controversies concerning the architecture of the myofibres in the right ventricular wall and the muscular ventricular septum. We used quantitative polarized light microscopy to obtain information about the orientation of myocardial cells in the ventricular mass. These cells, joined into a network by anastomoses, have at any point in the ventricular mass a principal direction--the fibre direction. We have quantitated this information in the form of maps of the azimuth and elevation angles, in 18 midgestation fetal hearts. Our findings show that the fibre architecture of the heart can be conceptualised as myocardial fibres running like geodesics on a nested set of warped "pretzels". This model is an extension to the whole ventricular mass of Krehl's Triebwerk, and Streeter's model which was restricted to the left ventricle. A "pretzel" itself can be considered as two doughnuts joined side-by-side, with the tunnel at the center of each doughnut corresponding to the ventricular cavity. Over and above the excellence of the fit between the data and the geodesic representation, three strong arguments support this model. First, it is the only existing model that explains the observed rolling over of fibres around the atrioventricular valvar orifices. Second, it explains the trajectory of the fibres from the epicardium to the endocardium at the basal parts of both ventricles and at the apical part of the left ventricle. Third, the predicted topological singularities of the model are systematically observed in each of the 18 hearts studied.