Animation of a human blastocyst, a hollow ball of cells that is formed around five days after fertilisation of an egg cell by a sperm. The blastocyst comprises a thin wall of cells called the trophoblast, and an inner cell mass (bottom) called them embryoblast. As the name implies, the embryoblast cells develop into the embryo. The trophoblast cells develop into the placenta, which connects the embryo to the uterine lining.