Four-cell embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the four cell stage, about 40 hours after fertilisation. Only three of the large embryo cells can be seen in this view, with smaller polar bodies seen at the centre. Still to form into a morula, a cluster of cells, each of these large rounded cells is called a blastomere. The cells are covered in microvilli. This embryo has only undergone two cell divisions and will continue to divide before implanting into the wall of the uterus (womb). They will grow to form a human foetus composed of many millions of cells. Magnification: x900 at 6x7cm size.
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