The blastocyst is a mass of cells which specifically forms five days after fertilization (in humans). “Blastos” is a Greek word which means “sprout” and “kystis” means “capsule”. This circular structure’s inner cell mass will eventually become the embryo and its outer layer will develop as the placenta. One day after its formation, the blastocyst travels down into the uterus and embeds itself into the uterine wall. This implantation is very important to the survival of the embryo as this process establishes its connection with the mother.