Human embryogenesis is a complicated process by which a fertilized egg develops into an embryo. During the first eight weeks of development, the conceptus shifts from a single-celled zygote into a multi-layered, multi-dimensional fetus with primitively functioning organs. The continued growth and increased intra-embryonic complexity during the first eight weeks of development are highly dependent upon cell signaling, proliferation, and differentiation. Due to the intricacies involved, the development of the human embryo is divided into developmental events by week. Week 1 is a major part of the germinal stage of development, a period of time that continues from fertilization through uterine implantation. Important events that occur during the first week of human embryonic development include gamete approximation, contact and fusion of gametes, fertilization, mitotic cleavage of the blastomere, morula formation, blastocyst formation, and implantation of the blastocyst.