 None of these processes occur in isolation and explanations of particular form features usually draw on several of them simultaneously, presuming other features that originated earlier in ontogeny by different instantiations and combinations of the processes. This sets a broad agenda for investigation: how do various iterations and combinations of these processes generate form features during ontogeny? Consider the concrete example of vertebrate cardiogenesis. How does the vertebrate heart, with its internal and external shape and structure originate during ontogeny (Harvey 2002)? How does the heart come to exhibit left/right asymmetry in the body cavity? What causes cells to adopt a muscle cell fate or certain tissues to interact in the prospective region of the heart? How do muscle cells migrate to, aggregate in, and differentiate at the correct location? How does the interior of the heart adopt a particular tubular structure with various chambers (that differs among vertebrate species)? How does the heart grow at a particular rate and achieve a specific size? Solutions relevant to explaining the ontogeny of form characterize causal factors that account for how different processes occur and yield various outcomes  (Section 3).