Name: prism stage
ECAO ID: 0000430
Definition: "The embryo is mainly characterized morphologically by a typical triangular shape, the beginning of the tripatition of the digestive tract and the presence of pigmented cells. The oral (or ventral) ectoderm has by now even more flattened, making an almost perfect right angle with the blastoporal side of the embryo. By contrast, on the opposite side, the aboral (or dorsal) ectoderm has round up and slightly elongated, thereby forming the primitive apex of the future larvae. These changes have conferred the embryo a typical and easy recognizable triangular shape. In the oral ectoderm, a small depression has further appeared, called the stomodeum, which will later fuse with the tip of the archenteron to form the mouth. In the blastocoel, the digestive tract has also by now bended toward the stomodeum and started to become tripartite. The cardiac sphincter has started forming, separating the future esophagus from the future stomach. The triradiate spicules have also continued elongating, thereby forming by now, towards the animal pole, the right and left dorsoventral connecting rods, towards the apex, the body rods, and along the vegetal, oral ectoderm the left and right ventral transverse rods. Moreover, some non-skeletogenic mesenchyme cells (or NSM cells, secondary mesenchyme cells or SMC) have started becoming pigmented and inserted into the aboral ectoderm. These cells represent part of the future immune system of the larvae along with other blastocoelar cells that are present in the blastocoel around the gut. In the vicinity of the future esophagus, other non-skeletogenic mesoderm descendant cells along with some endoderm cells have further arranged to form a single unpaired coelomic pouch. At the animal pole, the apical pole domain has also started to flatten making the embryo look like a square when viewed from the oral side, and it still arbors a tuft a long and immotile cilia. Finally, within the apical pole domain, but not yet in the ciliary band, molecular labeling surveys also start by this stage to reveal the presence of differentiating neuronal cells."
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