OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

When does a zygote become an embryo?

1. After implantation

2. At the 8-ball stage

3. At 14 days post-fertilization

4. When it splits into two cells

The correct answer is: At the 8-ball stage

The correct understanding is that a zygote becomes an embryo after implantation occurs in the uterus. The zygote is formed immediately after fertilization when the sperm and egg have fused; however, it undergoes several cell divisions as it travels down the fallopian tube before making its way to the uterus. At the 8-cell stage, the developing organism is still referred to as a zygote, and it has not yet implanted into the uterine tissue, which is essential for the transition to an embryo. It is after implantation that the zygote is classified as an embryo, as it establishes itself in the uterine lining and begins to undergo significant development. By 14 days post-fertilization, the developing structure is recognized as an embryo, and at that point, it begins to form fundamental tissues and organ systems. Therefore, the definition of the transition from zygote to embryo is closely associated with the implantation process. Dividing into two cells refers to the first division of the zygote, highlighting the initial stages of development but does not constitute the transition to an embryo.