Video - Human development week 2

At around day 8, the trophoblast gives rise to two layers; the cytotrophoblast and the syncytiotrophoblast. The cytotrophoblast which consists of mononucleated cells, makes the chorionic villi. On the other hand, the syncytiotrophoblast consists of multinucleated cells. These cells produce hCG (human chorionic gonadotropin) needed to keep the corpus luteum viable.

The corpus luteum needs to stay viable to keep producing progesterone needed to maintain the pregnancy until the placenta grows enough to take this task over. Also, the embryoblast differentiates into ventral hypoblast that makes the yolk sac, and the dorsal epiblast that later gives three embryonic germ layers.

Around days 9 to 12, the syncytiotrophoblast goes deeper into the decidua basalis for more nutrients needed to sustain growth, whereas the epiblast develops clefts that later coalesce to form the amniotic cavity. At around day 13, the hypoblast cells form the exocoelomic mesoderm cells outside the embryo. Finally, the epiblast gives rise to the three embryonic germ layers; endoderm, mesoderm, and ectoderm.