For the purposes of a reproducible morphologic boundary, we divided flank from ventral skin based on a change in skin thickness and divided dorsal from flank skin at the level of an ectodermal notch that lies at the same level as the ventral extent of the myotome (Figure 7) (Huang and Christ 2000; Olivera-Martinez et al. 2000; Sudo et al. 2001; Burke and Nowicki 2003; Nowicki et al. 2003).