This model is consistent with studies in the chick, where distinct dorsal and ventral lineage compartments exist for ectoderm in both the limb (Altabef et al. 1997, 2000; Michaud et al. 1997; Kimmel et al. 2000) and interlimb regions (Altabef et al. 1997, 2000), but not for limb mesoderm (Altabef et al. 1997; Michaud et al. 1997).