PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY PHILIPPE PLAILLY / SCIENCE PHOTO LIBRARY
Four-cell embryo. Light micrograph of an early, primitive stage of the human embryo, composed of only four cells. Each cell is called a blasto- mere. Granular cytoplasm can be seen inside the cells, and each has a central nucleus. Sperm with tails are visible around the embryo; these sperm were unsuccessful in reaching the egg in time to fertilise it. This embryo is a product of in vitro fertilisation (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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