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

At what stage can the embryo potentially split into twins?

1. Stage 3

2. Stage 4

3. Stage 5

4. Stage 6

The correct answer is: Stage 5

The correct stage at which the embryo can potentially split into twins is during the blastocyst stage, which corresponds to Stage 5 of embryonic development. At this stage, the embryo has divided into many cells and has formed a hollow structure known as a blastocyst. This is a point in development where the inner cell mass is formed, which can lead to the development of individual twins if it divides. When a single embryo splits into two separate embryos, this results in identical or monozygotic twins. This splitting can occur early in development before the cells begin to differentiate, making the blastocyst stage a critical point for the potential for twinning. In later stages of development, such as once the embryos have implanted and differentiated further, the likelihood of twinning decreases as the cells already have specific roles and can no longer easily form separate individuals. Therefore, Stage 5 is key for understanding the potential for embryos to split and form twins.