Fertilisation. Light micrograph of a human zygote at the point of fertilisation, produced by in- vitro fertilisation (IVF). A zygote is the cell produced when a sperm fertilizes an ovum. At this stage a sperm has entered the ovum, and the male and female pronuclei are seen making contact with each other (seen as clear circles within the zygote). Once united, the resulting zygote will contain both sets of genetic material from the mother and father. The cell will then divide into a ball of cells known as a blastocyst. In IVF, fertilisation is performed in culture medium and the blastocyst implanted directly into the uterus wall. Magnification: unknown.