What mass of cells forms a fluid-filled sphere during cleavage?

During early embryonic development, the mass of cells that forms a fluid-filled sphere is known as a blastocyst. After several rounds of cleavage, which is the process of rapid cell division following fertilization that does not involve growth, the resulting structure transitions from a solid ball of cells to a hollow sphere. This hollow portion is filled with fluid, leading to the characteristic blastocyst structure that consists of an outer layer of cells called the trophoblast and an inner cell mass that will eventually develop into the embryo.

The blastocyst stage is crucial as it facilitates implantation into the uterine wall, setting the foundation for further development. This stage occurs after cleavage but before the formation of other structures such as the embryo itself, ectoderm, or placenta. Each of these other options represents either a stage of development beyond the blastocyst or a specific tissue type that contributes to overall development but does not describe the hollow, fluid-filled sphere formed during cleavage.