Explain the events of gastrulation and the formation of 3 germ layers?

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. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers.

The purpose of gastrulation is to position the 3 embryonic germ layers, the endoderm, ectoderm and mesoderm. These layers later develop into certain bodily systems.

    * The ectoderm develops into the brain, skin, nails, the epithelium of the nose, mouth and anal canal; the lens of the eye, the retina and the nervous system.
    * The endoderm develops into the inner linings of the digestive tract, as well as the linings of the respiratory passages. It also forms many glands, such as the liver and pancreas.
    * The mesoderm forms the somites, the notochord, and the mesenchyme, which give rise to the muscles, circulatory and excretory systems of the body.