Light micrograph showing a hatching blastocyst, a six days-old human embryo. The blastocyst (left) at six days post-fertilisation is a hollow ball of cells with a fluid centre. Most of these cells will form the placenta & membranes, & only a small group (the inner cell mass) form the embryo proper. Here, the blastocyst is seen emerging from a hole in the zona pellucida (a protein shell" around the oocyte). At this stage in nature, the blastocyst would be in the uterus & ready to begin implanting into the endometrium (the uterine lining). This image was produced in the course of In Vitro fertilisation research."