The embryonic disc still consists of the epiblast and visceral hypoblast. The parietal hypoblast now lines the entire blastocystic cavity, giving rise to the primary umbilical vesicle. The uterine mucosa completely covers the implanted embryo except at the closing plug. The syncytiotrophoblast has developed strongly at the embryonic pole, forming lacunae, but is much thinner towards the abembryonic pole.