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. This is the first study to do so, using the gastropod Crepidula fornicata.Ĭrepidula gastrulation occurs by epiboly: the first through third quartet micromeres form an epithelial animal cap that expands to cover vegetal endomesodermal precursors. Different developmental explanations for this variation exist, yet no modern lineage tracing study has ever correlated the position of cells surrounding the blastopore with their contribution to tissues of the mouth, foregut, and anus in a spiralian. In this group, the blastopore has been said to become the mouth, the anus, both, or neither. Theories about the evolution of gastrulation often concern the fate of the blastopore (site of endomesoderm internalization), which varies widely in a major branch of bilaterians, the Spiralia. Gastrulation is a critical step in bilaterian development, directly linked to the segregation of germ layers, establishment of axes, and emergence of the through-gut.
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