Embryo development. Coloured scanning electron micrograph of an embryo at the early stage known as the morula. The egg reaches this phase about 4 days after fertilisation after a series of mitotic divisions. At this stage about 12-16 cells are present and are surrounded by a thin glycoprotein layer, the zona pellucida, which was here removed. The inner cells of the morula will give rise to the tissues of the embryo while the outer cells, covered here by microvilli (tiny yellow ridges), will form the placenta. The morula will implant into the uterus six days after fertilisation. Magnification: x1350 at 6x7cm size.