The outermost layer is thicker and in contact with uterine spaces. It becomes a syncitium (layer of nuclei and cytoplasm without lateral cell borders) and is termed the "syncytiotrophoblast". This outer layer continues to grow, eroding the endometrium and forming large vacuoles.Eventually these coalesce into lacunae. As the syncytiotrophoblast digs deeper into the endometrium, eventually (by 11 days) the endometrium encloses the embryo.