Blastocyst
blastocyst:http://www.med.yale.edu/obgyn/kliman/placenta/articles/EOR_Placenta/Trophtoplacenta.html

By 4-5 days after fertilization the embryo has differentiated into two distinct cell types: inner cell mass (lighter cells)–which will develop into the fetus and eventually become the newborn and trophoblasts (darker cells)–which will develop into the placenta and external membranes. Even by this stage the trophoblasts have begun to make their hallmark hormone: human chorionic gonadotropin (hCG)–the hormone of pregnancy-test fame.
Excerpted from: From Trophoblast to Human Placenta