![]() This connection is usually called either the ![]() The fate of this site is often not only the area of germ layer specification, it sometimes becomes the connection of the endodermal digestive cavity to the ectoderm and thus to the animal’s environ- ment. In most animals the site of internalization of gas- trulating cells is limited to a specific area, the blas- topore. Consequently, gastrulation plays a major role in hypotheses regarding the transition from a radi- ally symmetrical diploblastic animal to a bilaterally symmetrical triploblastic organism with meso- derm, the last common bilaterian ancestor.Ĥ.2 The blastopore as the site of internalization One can thus picture gastrulation asĪ fundamental event in every organism as it deter- mines the main body features during development. ![]() Gastrulation not only generates distinct cell types, it also establishes the organismal axes from a pre-existing animal–vegetal embryonic axis. Bilaterians have in addition a third germ layer, the mesoderm, which either separates from the endoderm or ingresses as independent precursors. During gastrulation, cells or cell layers are internalized and later form the digestive epithe- lium and often also germ cells. The developmental process that separates the inner from the outer cell populations is called gastrulation. The evolution of an internal germ layer enabled the compartmentalization of the body of multicellular animals (Metazoa) into a digestive layer (endo- derm) and an outer layer, the integument (ecto- derm). Here we review competing theories of bilaterian evolution and evaluate their plausibility in the light of recent insights into meta- zoan phylogeny and development.Ĥ.1 Gastrulation as a process determining multiple body plan characteristics A major difference between these theories concerns the structure of the alimentary canal and the relationship of its openings to the blastopore of the last common bilaterian ancestor. In addition to the for- mation of the germ layers it is often the time when the future axial properties and digestive open- ings become apparent, and it is not surprising that this event plays an important role in hypotheses regarding metazoan evolution. Gastrulation is one of the major events during the embryogenesis of an animal.
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