A great many contemporary embryologists are of the opinion that, in embryo development, a certain stage exists for the individual animal phyla in which all the representatives of a certain phylum are most similar. This is called the phylotypic stage. The phylotypic stage in vertebrates is the pharyngula stage, while in arthropods this is the segmented germ-band stage (Fig. XII.10). According to the original concepts, the phylotypic stage corresponds to the developmental stage in which the basic body structure is formed, a bauplan, which is characteristic for the given animal phylum.
The concept of the existence of a uniform phylotypic stage is probably somewhat simplified. Numerous heterochronies occur in the development of the individual species of organisms; i.e. the individual organ systems develop at different rates in various species (Palumbi 1997). It thus follows that the members of two taxa of a single phylogenetic line can be most similar at a certain stage in development, while the members of two other taxa are most similar in a different stage (Richardson et al. 1997; Richardson 1995).