Blastocyst Differentiation
The blastocyst is characterized by a group of cells, called the inner cell mass (also called embryoblast) and the mentioned trophoblast (the outer cells), and a blastocyst cavity (blastocoel).
The inner cell mass gives rise to the embryo proper, the amnion, yolk sac and allantois, while the trophoblast will eventually form the placenta. The blastocyst can be thought of as a ball of a (mostly single) layer of trophoblast cells, with the inner cell mass attached to this ball's inner wall. The embryo plus its membranes is called the conceptus. By this stage the conceptus is in the uterus. The zona pellucida ultimately disappears completely, allowing the blastocyst to invade the endometrium, performing implantation.
Read more about this topic: Human Embryogenesis