Human embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the 16-cell stage on the tip of a pin. The ball of cells (yellow) of the embryo is known as a morula, a cluster of almost identical, rounded cells, each containing a central nucleus. This 16-cell embryo is about three days old. It is at the early stage of transformation from a single cell to a human composed of millions of cells. The cells multiply by repeated cleavage divisions (mitosis) and will form a hollow ball of cells (the blastocyst). Development of the blastocyst occurs before the embryo implants into the wall of the uterus (womb). Magnification: x130 at 6x7cm size. Magnification: x450 at 8x10 inch size.