How is the movement of cell mass manifested during gastrulation? With what layer of cells does the embryo become three-layered?

In the process of gastrulation, the vegetative pole (lower part of the blastula) is invaded into the primary cavity of the body. Multicellular ones have a third layer – the mesoderm, which appears last during gastrulation, between the ecto- and endoderm. Then the embryo becomes three-layered.

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