The embryo is still in the Graaffian vesicle. Since the moment of fertilization, he has not changed much in appearance, although he has already formed extra-embryonic organs: chorion, amnion, yolk sac. All of them are necessary for further development. Also in the second week, cell nuclei and cytoplasm are formed. By the end of this period, the egg becomes mature: it has a nucleus, a fully formed cytoplasm, a transparent membrane and a crown of epithelial cells. The shell protects the tiny organism from damage and keeps it alive. Most likely, it will not be possible to examine the future baby on an ultrasound, since it is the size of a poppy seed.