Hatched blastocyst embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the blastocyst stage, six days after fertilisation. It has fully hatched from the zona pellucida (not seen), a protein shell that originally surrounded the unfertilised egg. The 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 in the uterus (womb) and is ready to implant on the endometrial wall of the womb. Magnification: x520 at 6x7cm size. Magnification;x480 at original 6x7cm size.