Four-cell embryo. Light micrograph of the blasto- meres of a four-cell human embryo, two days after fertilisation. The blastomeres are the rounded cells, here formed after two divisions of the fertilized egg. One cell is seen in front of the other three. An envelope, the zona pellucida, is seen surrounding the embryo, and the heads of some sperm are visible still attached to it. Magnification: x260 at 5x7cm size. x130 at 35mm size.