What are the pathways for the assembly of vertebrate organs, and how have these pathways evolved from those in more primitive organisms? Our goal here is to suggest one logical framework for approaching organ development, based upon recent genetic studies and buttressed by evolutionary observations. We do not presume the model to be as compelling as that established for pattern and polarity in the Drosophila melanogaster embryo, although we draw upon the logic of that system, in proposing that single gene mutations can reveal units of vertebrate organ assembly, particularly the heart. These units we refer to as “modules,” by which we mean regions or functions of the heart, such as chamber designation or localized pacemaker activity, which can be selectively perturbed by single gene mutations in mice or fish. The modules appear to be recent evolutionary additions, not present in primitive chordates, in contrast to myogenic cellular activity, which appears to be an ancient metazoan property with molecular pathways conserved from insect to mammal. We suggest, in a heuristic vein, that it may be possible to view vertebrate heart assembly as the sum product of separable genetic modules.