Specialists from Germany were able to follow the process of development of embryo rats in the first three days after joining of egg and sperm. As you know, the first population of cells is uniform. These cells are called stem from them are formed all the tissues and organs.
There is no difference between the initial stages of fetal development of mammals. It all starts with the blastula, the aggregate of cells forming a sphere. This structure is formed in mice and in humans. At the moment of fusion of egg and sperm determine the future of a fetus, genetic characteristics. Hard to imagine, but just one cell after a while the light appears a full-fledged living organism.