By one week after conception, the multiplying cells have divided into outer cells enclosing a mass of inner cells. The inner cells will become the:

By one week after conception, the multiplying cells have divided into outer cells enclosing a mass of inner cells. The inner cells will become the embryo.

Explanation:

By week one, the outer cells will give the trophoblast, which will later become the placenta, while the inner cells will form the embryoblast, which will become the embryo. Then the inner cells will be in one end, and in the other one, there would be the blastocele, which is a space with fluids. This process takes place in the fallopian tube and is part of the segmentation process.