The formation of the single heart tube by hypothetical fusion of two separately developed heart tubes is re-investigated, because this intricate process is ambiguously and often incompletely described. To gain a better insight into this problem ten mouse embryos ranging from 7.5 to 8.5 days of development (presomite to 6 somites) were serially sectioned (1 micron) and reconstructed graphically. Twenty mouse embryos of comparative ages, were studied by scanning electron microscopy. Two large embryonic mesodermal compartments, derived from the primitive streak, extend rostrally on either side of the embryonic axis, and meet in front of the buccopharyngeal membrane. In each compartment a coelomic cavity develops, splitting the mesoderm into a splanchnic and somatic layer. The splanchnic mesoderm differentiates into a layer of cuboidal splanchnic mesothelial cells (promyocardium) and a subjacent plexus of elongated endothelial cells (proendocardium). Before the 1-somite stage the left and right splanchnic mesoderm are separated in front of the buccopharyngeal membrane by a thickening of the yolk sac endoderm. The splanchnic mesoderm then fuses, forming a single horseshoe-shaped heart primordium consisting of a promyocardial layer and a subjacent vascular plexus. Until the 2-somite stage both coelomic cavities remain separated by a bilayer of squamous somatic mesothelial cells ('mesocardium'). The plexus of endothelial cells that forms the proendocardium, also seems to be the source of the lining of the vitelline veins, the pharyngeal arch arteries and the dorsal aortae. The relatively close adherence of endoderm to the medial part of the horseshoe-shaped heart primordium, combined with a bilateral accumulation of cardiac jelly, is suggestive of a double heart tube. However, promyocardium and proendocardium are both translocated as one horseshoe-shaped layer, thus fusion of the left and right parts of the heart primordium does not occur.