After a successful fertilization, which takes place in the ampullary part of the fallopian tube, the embryo migrates through the tube into the uterine cavity. This migration takes six days. Along the way, the zygote divides several times, initially without increasing its volume because it is still enveloped by the pellucid zone. Daughter cells are engendered and one speaks now of the blastomere stage. After around 16 cells (morula) the compaction occurs in which the outer cells, the trophoblasts, form a compact epithelial structure. They are bound to each other via tight junctions and microvilli are formed towards the outside. The inner cells are thus protected from the influences of the outer milieu and can differentiate in other ways. There, an embryoblast comes into being. At the same time a fluid-filled space, the blastocyst cavity, is formed. One speaks now of a blastocyst . Subsequently the blastocyst emerges (hatches) from the pellucid zone and now finds itself as a free blastocyst on the mucosa surface in the uterine cavity. Two layers of cells can now be distinguished in the embryoblast: the epiblast and the hypoblast. Thereafter the free blastocyst embeds itself with the pole where the embryoblast is located into the endometrium; this is termed adplantation. With enzymes of its thropholast the blastocyst dissolves the endometrium and slowly penetrates deeply into it. With this the implantation has begun.