PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY
Four-cell embryo. IVF (in vitro fertilisation) researcher examines a four-cell embryo through a light microscope. The embryo is displayed on the TV screen. This is an early, primitive stage of the human embryo, composed of only four cells. Each cell is called a blastomere. Granular cyto- plasm can be seen in the cells, and each has a central nucleus. Sperm with tails are seen around the embryo. This embryo is a product of IVF, that is artificial fertilisation in the laboratory. At the four-cell stage the embryo has divided twice by mitosis, and must grow further before it can implant in the uterus.
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