The other cells in a blastocyst, besides those that will become the placenta, will become the embryo itself.

Here's a breakdown:

* Blastocyst: A hollow ball of cells that forms about 5-7 days after fertilization.

* Trophoblast: The outer layer of cells in the blastocyst. These cells will develop into the placenta, which provides oxygen and nutrients to the developing embryo and removes waste products.

* Inner cell mass (ICM): The cluster of cells inside the blastocyst. These cells will develop into the embryo, including all its organs, tissues, and systems.

So, while the blastocyst contains cells that will form both the placenta and the embryo, the cells that ultimately become the embryo are found in the inner cell mass.