During the first two weeks (the pre-embryonic period) the embryo moves through the uterine tube to the uterus where it will implant. The differentiation that establishes the organ systems takes place between 3 and 8 weeks in the first 8 weeks following fertilization (the embryonic period).
First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes.
