@Nefertini - I agree it's a fascinating process with different stages happening every week during the development of the embryo. Gastrulation, for example, is the phase in human embryonic development in which the single-layered blastula forms the tri-layered gastrula. The gastrula consists of the endoderm, mesoderm, and ectoderm. In some organisms, the gastrula only consists of two layers - the endoderm and ectoderm. In humans, gastrulation occurs during the third week of embryonic development.