Gastrulation accomplishes several tasks that are critical for metazoan development, most importantly the segregation of germ layers and re-arrangement of cells to form the basic body-plan. The anus forms days later, as a secondary opening within the 2d 2 clone, and not from the classically described “anal cells”, which we identify as the 3c 221 and 3d 221 cells. Progeny of 2a-2c and 3a-3d make the mouth and foregut, and the blastopore becomes the opening to the mouth. During this process, several of these cells, as well as the 2d clone, become displaced posteriorly, away from the blastopore. Posteriorly, cells derived from 3c 2 and 3d 2 undergo a form of convergence and extension that involves zippering of cells and their intercalation across the ventral midline. These cells make a novel spiralian germ layer, the ectomesoderm. ![]() Anteriorly, cells derived from 3a 2 and 3b 2 undergo a unique epithelial-to-mesenchymal transition involving proliferation and a collective movement of cells into the archenteron. As the blastopore narrows, the micromeres’ progeny exhibit lineage-specific behaviors that result in certain sublineages leaving the lip’s edge. Initially, descendants of the second and third quartet micromeres (2a–2d, 3a–3d) occupy a portion of the blastopore lip. Crepidula gastrulation occurs by epiboly: the first through third quartet micromeres form an epithelial animal cap that expands to cover vegetal endomesodermal precursors.
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