Name: early gastrula stage
ECAO ID: 0000390
Definition: "The embryo is mainly characterized morphologically by the presence in the blastocoel of a small archenteron (i.e. primitive digestive tract) that is about a third of its finally size. Following the initial invagination of the vegetal plate, the archenteron has slightly been extended, within the blastocoel, by the subsequent invagination of additional non-skeletogenic mesoderm cells and of some endoderm cells. In parallel, the skeletogenic mesenchyme cells (or SM cells, primary mesenchyme cells or PMC) have by now started migrating along the blastocoel wall, towards the animal pole, thereby creating two lateral chains (one left and one right), while around the developing archenteron, they have gathered into two ventrolateral clusters separated by an oral (or ventral) and an aboral (or dorsal) chain defined by the remaining skeletogenic mesenchyme cells. Within the oral and aboral skeletogenic mesenchyme chains, the cells have further started to fuse to form syncytial filopodial cables within which skeletal calcification will subsequently take place. At the animal pole, the animal neuroectoderm territory is still marked by an enlarged epithelium bearing long and immotile cilia."
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