ZEPHYR / SCIENCE PHOTO LIBRARY ZEPHYR / SCIENCE PHOTO LIBRARY
IVF treatment. Light microscope of human egg cells (large, round) and sperm (small dark ovals with tails) in a petri dish during in vitro fertilisation (IVF). The eggs and sperm are incubated together in a culture medium for around 18 hours. For each egg roughly 75,000 sperm cells are added to the medium. Any eggs that are fertilised at the end of the 18 hour period will be grown in the lab until they reach an early stage of embryonic development. They are then checked and implanted in the patient's uterus.
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