Four sequential stages offertilization are shown, beginning with a mature, but unfertilized human egg (top left). Next comes the post-fertilization incorporation of a sperm pronucleus, which in this example is moving inward from the left (top right).This stage is followed by the fusion of the egg and sperm pronuclei (bottom left), which in turn leads to the alignment of the mixed chromosomes from both parents on the first mitotic spindle (bottom right). (From C. Simerly et al., Nat. Med. 1:47–53. © Macmillan Magazines Ltd.)