FREEMAN, G.; Univ. Texas: The Evolutionary Significance of Early Development

Many features of the body plans of animal phyla are blocked out during early development even though additional body plan altering events can occur later during development of animals with complex life cycles. These developmental periods are frequently referred to as ‘phylotypic stages’; this isa term that has become misleading. Prior to the blocking out of the body plan of an animal a number of regional specification events occur that are responsible for establishing the the axial properties of the embryo, the origin and differentiation of germ layers that make it up and the placement of different body region of an embryo along its axes. These cellular elements and the patterns of localized differential gene activities that allow them to occur are the elementary building blocks that will have to be used to find out to what extent animals that belong to the same phylum set up their initial body plan in the same way. They are also the basis for comparing differences in patterns of regional specification between species.