DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Two-cell embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the two cell stage, about 30 hours after fertilisation. Still to form into a morula, a cluster of cells, each large rounded cell here is called a blastomere. The smaller spherical structure (at lower centre) is a polar body which will degenerate. The surface of each cell is covered in microvilli. This embryo has only undergone one cell division. The cells will continue to divide and eventually will transform into a human foetus composed of millions of cells. At this two-celled stage, the embryo has not yet implanted in the uterus (womb). Magnification: x830 at 6x7cm size.
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