Hatching blastocyst embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the blastocyst stage, five days after fertilisation. The embryo (blue) is hatching from the zona pellucida (green), a protein shell that originally surrounded the unfertilised egg. Sperm tails (red) are also visible. A blastocyst is a hollow ball of cells with a fluid centre. Each cell is called a blastomere. Most of these embryo cells will form the placenta and membranes around the embryo, and only a small group (the inner mass) form the embryo proper. At this stage the blastocyst is preparing to implant on the wall of the uterus (womb). Magnification: x1030 at 5x7cm size. Magnification: x3700 at 10x6.5 size."