What is the term for an embryo at the stage of implantation consisting of a fluid-filled ball of cells?

The correct term for an embryo at the stage of implantation that is characterized by a fluid-filled ball of cells is the blastocyst. This stage occurs approximately five to six days after fertilization and is crucial for the process of implantation into the uterine lining.

The blastocyst consists of an inner cell mass, which will eventually develop into the embryo, and an outer layer called the trophoblast, which will contribute to the formation of the placenta. As it implants, the embryo begins to establish a connection to maternal blood supply, which is essential for its further development.

In contrast, a blastula is an earlier stage that precedes the blastocyst stage and is not specifically adapted for implantation. The gastrula represents a later stage in embryonic development where the three germ layers (ectoderm, mesoderm, and endoderm) are formed, leading to differentiation and organ development. A fetus refers to a stage that occurs much later in development, after major structures have already formed, making it distinct from the blastocyst state.