PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Blastocyst embryo. Light micrograph of a human embryo at the blastocyst stage. This early blastocyst is about four days old, appearing as a hollow ball of cells. On around day six, the embryo will begin to implant into the wall of the uterus. Magnification unknown.
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