The process is likely to be triggered by the formation of the layers. Zygote is the structure that forms when a sperm fertilizes an egg, the zygote then divides many times by the process of mitosis to form an embryo. That is, it forms two cells, then four, then eight and so on, up to the eight stage the cells are identical and are called embryonic stem cell, which then develop into any specialized cells type by differentiation, that the growing embryo needs, such as the nerve cells, blood cells, among others.