| The early stages of organogenesis in metazoans differ drastically between higher order taxa such as phyla and classes. The segmented germ band stage in insects, the nauplius stage of crustaceans, and the neurula/pharyngula stage in vertebrates are examples of this diversification. In striking contrast with this divergence, is the similarity of these stages within these taxa, i.e., within insects, crustaceans, and vertebrates. The early stages of organogenesis, or phylotypic stages, have, thus, remained very similar in most species since the evolutionary origin of the taxa. These phylotypic stages are considerably more similar to each other than to the earlier stages of cleavage and gastrulation. Cleavage and gastrulation stages display not only great... |
