Name: mid-gastrula stage
ECAO ID: 0000400
Definition: "The embryo is mainly characterized morphologically by the presence of an archenteron (i.e. primitive digestive tract) that has reached one-half its way into the blastocoel. A second phase of archenteron elongation has taken place by invagination of additional cells into the blastocoel, by cell division or by convergent-extension movements depending on the sea urchin species. Non-skeletogenic mesoderm cells that are present at the tip of the archenteron have further started ingressing into the blastocoel, generating the non-skeletogenic mesenchyme cells (or NSM cells, secondary mesenchyme cells or SMC). The oral ectoderm has also started to flatten and to become thicker, providing the first morphological indicator of the oral-aboral (or ventro-dorsal) axis of the embryo. As a consequence the oral chain formed by a portion of the skeletogenic mesenchyme cells has also flattened. Finally, skeletal calcification has further begun within the skeletogenic mesenchyme ventrolateral clusters, marked by the presence of the first calcified structures, which appear as small dots at that stage. At the animal pole, the animal neuroectoderm territory is still marked by an enlarged epithelium bearing long and immotile cilia."
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