During what stage does the embryo become a blastocyst?

The embryo becomes a blastocyst during the process of blastulation. This stage follows cleavage, where the fertilized egg undergoes a series of rapid cell divisions without significant growth, resulting in a solid ball of cells known as the morula. As development progresses into blastulation, the morula transforms into a hollow structure called the blastocyst. This structure comprises an inner cell mass, which will develop into the embryo, and an outer layer of cells known as the trophoblast, which will contribute to the placenta and support the developing embryo.

This key transition represents an important step in early embryonic development as it prepares the embryo for implantation in the uterine wall, facilitating further development and differentiation as it continues to grow. The other stages mentioned—fertilization, cleavage, and gastrulation—occur sequentially but do not specifically represent the formation of the blastocyst.