 One response to this criticism is to emphasize the conserved genetic mechanisms shared by all animals despite differences in developmental phenomena (Gerhart and Kirschner 2007; Ankeny and Leonelli 2011; Weber 2005). Fruit flies may be unrepresentative in exhibiting syncytial development, but they use the collinear expression of Hox genes to specify their anterior-posterior body axis. This response indicates that whether an entire model organism is representative per se is too coarse-grained a criterion to capture the rationale behind their use. We have to ask about representation with respect to what, and some accounts have moved in this direction. Jessica Bolker has distinguished exemplary and surrogate modes of representation (Bolker 2009), where the former serve basic research by exemplifying a larger group and the latter correspond to models designed to provide indirect experimental access to otherwise inaccessible phenomena, such as mouse models of human psychological disorders (e.g., depression). Surrogate models are adopted in biomedical contexts where the phenomena of interest are manifested in humans. Most developmental biologists consider model organisms as exemplars, not  surrogates.[15]  Thus, in order to respond to a criticism of non-representativeness, the criterion of representation must be explored in more  detail.[16]