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1 - blastula, 2 - gastrula; orange - ectoderm, red - endoderm.

Gastrulation is a phase early in the development of animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration. Gastrulation varies in different phyla; the following description concerns the gastrulation of the echinoderms, representative of the triploblasts, or animals with three embryonic germ layers.

At the beginning of gastrulation, the embryo is hollow ball of cells known as the blastula, with an animal pole and a vegetal pole. The vegetal pole begins to flatten and then invaginates into the interior, replacing the blastocoelic cavity and thereby forming a new cavity, the archenteron (literally: primitive gut), the opening into which is the blastopore. Some of the cells of the vegetal pole detach and become mesenchyme cells. The mesenchyme cells divide rapidly, migrate to different parts of the blastocoel, and form filopodia, strands that help to pull the tip of the archenteron towards the animal pole. Once the archenteron reaches the animal pole, a perforation forms, and the archenteron becomes a digestive tract passing all the way through the embryo.

The three embryonic germ layers have now formed. The endoderm, consisting of the archenteron, will develop into the digestive tract. The ectoderm, consisting of the cells on the outside of the gastrula that played little part in gastrulation, will develop into the skin and the central nervous system. The mesoderm, consisting of the mesenchyme cells that have proliferated in the blastocoel, will become all the other internal organs.

Gastrulation is followed by the organogenesis, during which the individual organ anlagen of the embryo are set up within the newly formed germ layers. Part of the organogenesis is the Neurulation.

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