A schematic representation of the processes that occur during the formation of the primitive heart tube. The schematic is modeled after the description given by Kaufman and Bard (1999). (A) The coelomic epithelial cells that are destined to form the heart tube. (B) The coelomic epithelial cells have formed the cuboidal cells of the cardiogenic plate. (C) The plate is undergoing morphogenesis to form the primitive heart tube. (D) The heart tube is complete, and cells have differentiated to give rise to the endocardium, cells forming the cardiac jelly, and the myocardium. For illustrative purposes, the colors of this figure correspond to the colors in each of the text-based graphs of Figures 2 and 8.