The zygote is the first diploid cell that forms following fertilization by fusion of the haploid oocyte (egg) and spermatozoa (sperm) resulting in the combination of their separate genomes. The zygote will therefore form the concepts, the embryonic (embryo, fetus) and extra-embryonic cellular products of fertilization. The first image shows the cell with the 2 pronuclear still present before fusion enclosed within the zona pellucida.
These two pronuclear contain the parental genomes and are reprogrammed separately, and also have different epigenetic changes at this early zygote stage. In humans, zygote stage of development occurs during the first day in week one following fertilization(GA week 3) and is described as Carnegie stage. This stage is followed by mitosis to form 2 blastomeric and then a solid cell mass called the morulla.