Deuterostomy is a condition in which the blastopore was retained as the bilaterian anus, the mouth developing as a secondary opening.Protostomy is a condition in which the blastopore was retained as the bilaterian mouth, the anus developing as a secondary opening.Amphistomy is a condition in which the tubular gut evolved from the sack‐shaped gut through lateral blastopore closure, leaving mouth and anus.The tubular gut of the bilateral animals evolved from the sack‐shaped gut of the common ancestor.The latest common ancestor of the eumetazoans (ctenophores and placozoans are not discussed here) was a gastrula‐like organism, with a sac‐shaped gut with a blastopore.It is concluded that the tubular gut with mouth and anus most probably evolved through amphistomy. fate of the actual blastoporal opening fate of the tissues surrounding the blastoporal opening, studied both through cell‐lineage and gene expression morphology and embryology of the central nervous systems and morphology of larval ciliary bands according to the trochaea theory. A recent review has discussed the most informative characters related to the blastopore fates, viz. Three theories for the evolution of the tubular gut prevail: (1) Protostomy in which the blastopore should become the mouth and the anus develop secondarily, (2) Deuterostomy in which the blastopore should become the anus and the mouth develop secondarily and (3) Amphistomy in which the blastopore should become divided into mouth and anus through fusion of the lateral blastopore lips. Cell‐lineage studies show that gastrulation through epiboly and invagination follow similar patterns with the cells of the blastopore rim bordering the cells which give rise to endo‐mesoderm. These data provide the necessary framework for ongoing investigations into the molecular basis of gastrulation and fate specification of mouth and anal tissues, which are essential for understanding basic origins of metazoan body plans.The bilaterian tubular gut with mouth and anus is generally believed to have evolved from the sack‐shaped gut of a gastrula‐like organism. The anus is derived from the 2d lineage that generates a remote clone located at the posterior end of the embryo and never contributes to the blastopore. Live imaging and lineage tracing showed definitively that the cells ringing the blastopore (derived from 2a-c and 3a-d micromeres) form the mouth in Crepidula. However, lineage tracing is necessary to confirm the fate of cells rimming the blastopore, as cell rearrangement there is highly dynamic. In some species it has been argued that the blastopore gives rise to both mouth and anus. While in most deuterostomes the blastopore becomes the anus, in protostomes the blastopore can be the site of anus or mouth formation. A controversial aspect of gastrulation is the fate of the blastopore (the site of endoderm formation).
These data allow us to directly compare epiboly between Crepidula and other clades. First we characterized the epibolic spreading of the micromere cap as it envelops the mesentoblast and other endodermal cells. Here we summarize results from an investigation of gastrulation mechanisms in the snail Crepidula fornicata, a model system for development among lophotrochozoans. However, lophotrochozoan gastrulation has been studied in very few species, despite its importance for understanding the evolution of this process. Modes of gastrulation (e.g., ingression, invagination, epiboly) are particularly diverse among the bilaterian protostomes. 5 10:30 Gastrulation and the fate of the blastopore in the snail Crepidula fornicata LYONS, D/C* PERRY, K HENRY, J/Q Duke University University of Illinois, Urbana-Champaign is a critical morphogenetic process in metazoan development by which presumptive endoderm and mesoderm cells become internalized.
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