Gastrulation

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.

Quotes

 * It is not birth, marriage or death, but gastrulation which is truly the most important time in your life.
 * Lewis Wolpert, quoted in J.M.W. Slack (ed.) From Egg to Embryo: Determinative Events in Early Development (1999), Ch. 1 : Regional specification in animal development