PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Four-cell embryo. Light micrograph of a 4-cell human embryo. The cells, or blastomeres, result from two divisions of the fertilized egg. This stage is normally reached around 40 hours after fertilisation. The cells are surrounded by the zona pellucida layer. Outside this at lower right are cumulus corona cells (small circles), the remains of the corona radiata which supported and nourished the egg while it was in the ovary. Photographed at the Hospital Cochin, Paris, France. Magnification unknown.
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