Four-cell embryo. Image 2 of 4. Light micrograph of the cells (blastomeres) of a 4-cell human emb- ryo 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. Cytoplasmic fragments may be left around the cells (inside the zona pellucida) by cell divisions, which decreases the embryo's chances of survival. This embryo has a few fragments (small rounded areas) around it, but will still be impla- nted. See images P680/488-491 for embryos with su- ccessively more fragments. Magnification unknown.
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