Which description best characterizes the blastocyst stage?

• A. Stage of early development consisting of a solid ball of cells
• B. Stage of early development that occurs 2-3 days after fertilization
• C. Stage of early development in mammals that consists of a hollow ball of cells (5-6 days after fertilization)
• D. Stage of early development where the embryo implants

Answer: (C)

The idea being tested is recognizing the defining structure and timing of the blastocyst. The blastocyst is characterized by being a hollow ball of cells with a fluid-filled cavity called the blastocoel. This hollow stage marks the transition from the earlier solid cluster of cells (the morula) to a structure ready to implant, and it typically forms in mammals about five to six days after fertilization. While implantation into the uterine lining begins during this stage, the essential description is the hollow nature of the blastocyst. The other descriptions don’t fit as well: a solid ball of cells describes the earlier morula, and the stated timing is earlier than the blastocyst stage, while the implantation note refers to a process rather than the defining structure of the stage.