GALIS, F.: Modularity and the conservation of characters determined in the phylotypic stage

The phylotypic stage is the developmental stage at which vertebrates most resemble each other. In an earlier study we found support for Raff’s hypothesis that the remarkable conservation of the phylotypic stage in vertebrates is due to the high number of ongoing interactions. Interestingly, the lack of modularity of inductive interactions at that stagemay not only have conserved this embryonal stage more than other developmental stages, but may also have had consequences for the conservedness of adult characters determined in this stage. Possible examples are the number of digits in amniotes and the number of cervical vertebrae in mammals. Evidence will be discussed. I end by presenting an argument why the absence of modularity in the inductive interactions may also be the root cause of the conservation of the much discussed temporal and spatial colinearity of the Hox genes in the phylotypic stage of vertebrates.