The blastopore forms the anus of the alimentary system in the juvenile and adult forms. The archenteron develops into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the blastopore of the gastrula. The resulting coelom is termed an enterocoelom. This happens when the mesodermal pouches become separated from the invaginating endodermal layer forming the archenteron, then expand and fuse to form the coelomic cavity. In deuterostomes, the endodermal lining of the archenteron usually forms buds called coelomic pouches that expand and ultimately obliterate the embryonic blastocoel (the cavity within the blastula and early gastrula) to become the embryonic mesoderm, the third germ layer. There are a series of other developmental characteristics that differ between protostomes and deuterostomes, including the type of early cleavage (embryonic cell division) and the mode of formation of the coelom of the embryo: Protosomes typically exhibit spiral mosaic cleavage whereas deuterostomes exhibit radial regulative cleavage. In protostomes (“mouth first”), the first embryonic opening becomes the mouth, and the second opening becomes the anus. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the mouth develops as a secondary structure opposite the location of the blastopore, which becomes the anus. Recall that protostomes and deuterostomes differ in certain aspects of their embryonic development, and they are named based on which opening of the archenteron (primitive gut tube) develops first. “This is why evolutionary biologists have given so much emphasis to the fate of the blastopore in animal evolution, and why it was so important to attain a better understanding of the developmental mechanisms controlling this embryonic process,” Jose Maria Martin-Duran points out.The phyla Echinodermata and Chordata (the phylum that includes humans) both belong to the superphylum Deuterostomia. Historically, the different fate of the blastopore has been a major feature to classify bilaterally symmetrical animals, that is, those with a head and a tail, and a back and belly.Īlready in 1908, the Austrian zoologist Karl Grobben proposed that bilaterian animals should be classified according to whether they form the mouth from the blastopore (Protostomia literally “first mouth”) or the anus (Deuterostomia “secondary mouth”).Īlthough the researchers now use other methods to unravel the evolutionary relationship between animal groups, the division of Protostomia and Deuterostomia proposed by Grobben has demonstrated to be correct, with only some minor modifications. “Importantly, none of our findings gave support to any of these traditional explanations,” says Martin-Duran. On the other hand, the Planuloid-Acoeloid scenario proposes that the blastopore was originally the mouth, and that the formation of the anus from the blastopore evolved secondarily. The first scenario is the Amphistomy concept, which assumes that the blastopore was originally both mouth and anus and the formation of only one or the other gut opening from the blastopore evolved later. Two major explanations have been proposed, both of them using present embryos as proof of ancestral animal forms, which is an idea strongly influenced by the famous German zoologist Ernst Haeckel and his principle that development recapitulates animal evolution. The different fates of the blastopore have been recognized for over a century. The study is recently published in the new journal Nature Ecology and Evolution. "There is no necessary connection between the mouth, the anus and the blastopore, neither during the development of animals nor as an evolutionary step in the past." “One of the most important conclusions of our work is that there is no necessary association of the mouth and the anus with the embryonic blastopore," says Martin-Duran. It is not a predefined attribute of the species, as previously thought,” says postdoctoral researcher Jose Maria Martin-Duran, at the Hejnol Group at Sars Centre at the Department of Biology, University of Bergen (UiB). “Our findings demonstrate that whether the blastopore forms the mouth or the anus is a consequence of how each embryo is organized during early development. How this happens has not been clear until now. By Kim Einar Andreassen Updated: (First published: )Īnimals often form either the mouth or the anus from an opening that appears in the early embryo, which is called the blastopore.įor instance, starfish develop the anus from the blastopore, but earthworms form the mouth out of it.
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