heart development trait sinus venosus morphology trait Any measurable or observable characteristic related to the shape, structure, color or pattern of the cavity at the caudal end of the embryonic cardiac tube in which the veins from the intra- and extraembryonic circulatory arcs unite; in the adult this becomes the portion of the right atrium known as the sinus of the vena cava. VT:0003228 Trait.ontology