Light micrograph (phase contrast) of a primitive human embryo, composed of four cells following the initial mitotic divisions which ultimately transform a single cell organism into one composed of millions of cells. Subsequent division of these cells (called blastomeres) results in the formation of a hollow ball of cells (the blastocyst) with a localised thickening (the inner cell mass) that will develop into the actual embryo. Development of the blastocyst occurs before it implants into the uterine wall. The site of implantation determines the subsequent position of the placenta.