PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Two-cell embryo. Light micrograph of a 2-cell human embryo. The cells, or blastomeres, result from the first division of the fertilized egg, which normally occurs 30 hours after fertilisation. They are surrounded by the zona pellucida layer. Outside this are cumulus corona cells (small circles), the remains of the corona radiata that offered the egg support and nutrition when it was in the ovary. Photographed at the Hospital Cochin, Paris, France. Magnification unknown.
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