DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY DR YORGOS NIKAS / SCIENCE PHOTO LIBRARY
Sixteen-cell embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the sixteen cell stage, four days after fertilisation. Known as a morula, this is a cluster of large rounded cells called blastomeres. The surface of each cell is covered in microvilli. Smaller spherical structures seen will degenerate. This embryo is at the early stage of transformation into a human composed of millions of cells. Here it is in the process of dividing to form a hollow ball of cells (the blastocyst). At this sixteen-cell stage the morula has not yet implanted in the uterus (womb). Magnification: x620 at 6x7cm size. Mag.: x2,300 at 8x10"; x960 at 4x5" size.
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