The zygote undergoes a series of cell divisions called cleavage, which results in the formation of a ball of cells called a blastocyst. The blastocyst consists of an inner cell mass that will develop into the embryo, and an outer layer of cells that will form the placenta. The blastocyst travels down the fallopian tube and enters the uterus, where it will eventually implant into the uterine wall. Implantation occurs when the blastocyst attaches to the endometrium, a process that is facilitated by specialized cells in the blastocyst called trophoblasts. Once implanted, the trophoblasts begin to invade the endometrium and form the placenta.