Human embryo. Light micrograph of a human embryo shortly after fertilization. Surrounding the embryo is the zona pellucida, a spongy layer that originally surrounded the unfertilized egg cell. The cells inside the embryo, called blastomeres, will continue to divide until they form a hollow ball of cells called a blastocyst, part of which will develop into the foetus. Formation of the blastocyst occurs before the embryo implants into the wall of the uterus. Note the sperm cells embedded in the zona pellucida. Magnification unknown.