Four-cell embryo. Image 1 of 4. Light micrograph of the cells (blastomeres) of a four-cell human embryo two days after in vitro fertilisation (IVF). In IVF, sperm is introduced to the egg in the laboratory. An envelope (zona pellucida) is seen around the embryo. Embryos are checked at this stage to ensure they are suitable for implan- tation into the mother's uterus. Cell divisions may cause some cytoplasmic fragments to be left around the cells, inside the zona pellucida, which decreases the embryo's chances of survival. This embryo has very few fragments around it. See images P680/488-491 for embryos with successively more cytoplasmic fragments. Magnification unknown.