This structure is called a blastocyst. It is made up of about 100 cells and has two distinct parts: the inner cell mass, whichdevelops into the embryo, and the outer layer of cells, which develops into the placenta and other support structures for the embryo. The blastocyst is the stage at which the embryo implants into the lining of the uterus, where it will continuewill eventually develop into the embryo, and the outer layer known as the trophectoderm, which will form the placenta and other support tissues for the growing embryo. The blastocyst is important because it is what implants into the lining of the uterus and initiates the process of pregnancy.