blastocyst
/ˈblæstəˌsɪst/
noun
- A very early stage of an embryo, consisting of a hollow ball of cells with an inner cell mass, which implants into the uterus.
- Cells from the blastocyst's inner mass will develop into the fetus.
- In IVF, the embryo is often transferred at the blastocyst stage.
- The blastocyst forms about five days after fertilization.