The development of human embryo starts with fertilization of the ovum. Fertilization normally occurs in the outer third of the Fallopian tube. Immediately following fertilization, the fertilized egg starts to divide. At the two cell stage, the embryo is called two-cell stage blastomere. This division continues as the fertilized egg is being passively transferred to the uterine cavity. Such movement is due to the mucociliary action of the epithelial lining of the Fallopian tube and to some extent due to its contraction. The constant division of the ovum leads first to the formation of morula. Around day 4 after fertilization, the morula arrives to the uterine cavity. Shortly after its arrival to the uterus, some fluid-filled spaces appear in the central regions of the conceptus which is now called blastocyst. During this free floating stage, the blastocyst is nourished by the secretions of endometrial glands. A day or two later, the embryo implants in the endometrium, normally in the upper and posterior part of the uterine cavity. The differentiation of cells in the blastocyst give rise to the parts that ultimately form placenta and embryo. The cells in the outer part of the blastocyst form the trophoblastic layer and those which are centrally placed give rise to the embryo. Further differentiation of the trophoblasts leads to the formation of two cell layer. The outer layer is called the syncytiotrophoblast and the inner layer is called cytotrophoblasts. These cells have the ability to invade the endometrial tissue so that by the end of the first week, the blastocyst superficially position itself in the wall of the uterine cavity. By day 9 after fertilization, amniotic cavity starts to appear and is covered by amnion cells which are derived from the cytotrophoblasts.