Final answer:
A few days after fertilization, the blastocyst forms, which is a hollow ball of cells that will eventually implant itself into the lining of the uterus.
Step-by-step explanation:
After fertilization, the zygote undergoes a series of divisions to form a structure called the blastocyst. This occurs a few days after fertilization. The blastocyst is a hollow ball of cells that will eventually implant itself into the lining of the uterus. It contains an inner cell mass that will develop into the embryo.