At the beginning of the second week the blastocyst has submerged in the endometrium During implantation the trophoblast develops into the syncytiotrophoblast and the cytotrophoblast which form finally the placenta
The embryoblast forms the epiblast while the trophoblast gives rise to the hypoblast The hypoblast forms the tissue of the primary yolk sac. The epiblast forms the three germ layers.
These germ layers, ectoderm mesoderm and endoderm develop through the primitive streak and form the actual embryo.
