Name: late gastrula stage
ECAO ID: 0000410
Definition: "The embryo is mainly characterized morphologically by the presence in the blastocoel of a fully extended archenteron (i.e. primitive digestive tract) that has now reached the roof of the blastocoel. The third and last extension phase of the archenteron has been ensured by pulling forces exerted by non-skeletogenic mesoderm cells that are still present at the tip of the archenteron. These cells have produced filopodia that extended towards the animal pole, attached the wall, and then shortened, thereby pulling up the archenteron. In some sea urchin species, this pulling force is already orienting the tip of the archenteron towards the oral ectoderm, where the future mouth of the larvae will form. In parallel, in the blastocoel, additional non-skeletogenic mesoderm cells continued ingressing from the tip of the archenteron and the skeletal calcified structures present in the skeletogenic mesenchyme ventrolateral clusters have further grown, forming now the primary triradiate skeletal rudiments (or spicules). At the animal pole, the animal neuroectoderm territory is still marked by an enlarged epithelium bearing long and immotile cilia."
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