Four-cell embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the four cell stage, about 40 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 structures will degenerate. The surface of each cell is covered in microvilli. This embryo has only undergone two cell divisions. The cells will continue to divide and eventually will transform into a human foetus composed of millions of cells. At this four-celled stage, the embryo has not yet implanted in the uterus (womb). Magnification: x680 at 6x7cm size. Mag; x625 at original 6x7cm size.