 Developmental biologists often speak of investigating mechanisms that account for phenomena in ontogeny (see  Section 1.3),  and focus on conserved genetic and cellular mechanisms in model organisms (Gerhart and Kirschner 2007; Ankeny and Leonelli 2011; Weber 2005). This suggests a distinction between representation with respect to developmental phenomena and representation with respect to genetic and cellular mechanisms operating in development. If we are interested in explaining how hearts (phenomena) develop, then we might investigate the molecular or cellular mechanisms occurring in the heart field during zebrafish ontogeny. Some of these mechanisms could be conserved even though the phenomena are not. Drosophila has only one cardiac cell type, no neural crest cells, and a heart with no atrial or ventricular chamber morphology (Kirby 1999). However, cardiogenesis in all invertebrates and vertebrates investigated thus far depends essentially on the expression of the homeobox gene Nkx2-5/tinman (Gajewski and Schulz 2002). The reverse situation also can hold: similar phenomena may be manifested but genetic and cellular mechanisms might differ. Amphibians form a neural tube (neurulation) through a process of invagination (the folding of an epithelial sheet), whereas teleost fishes form a neural tube via cavitation (the hollowing out of a block of tissue via cell death). The neural tube is homologous across vertebrates (i.e., a conserved phenomenon), but the cellular and genetic mechanisms involved in invagination versus cavitation are distinct (Davies 2013: ch. 4).