We show that cells of the dorsal aorta, an early blood vessel, and of the myotome, the first skeletal muscle to form within the somite, derive from a common progenitor in the mouse embryo. This conclusion is based on a retrospective clonal analysis, using a nlaacZ reporter targeted to the alpha-cardiac actin gene. A rare intragenic recombination event results in a functional nlacZ sequence, giv...