Human Embryogenesis

Human embryogenesis is the process of cell division and cellular differentiation of the human embryo that occurs during early stages of development. From a biological standpoint, human development is a continuum, starting with the germ cells (ovum and spermatozoon), through fertilization, prenatal development, and growth to adulthood. The germinal stage, refers to ovum (egg) prior to fertilization, through the development of the early embryo, up until the time of implantation.

During this stage, the fertilization creates a single-celled zygote, which is defined as an embyro because it contains a full complement of genetic material. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring either 8 weeks after fertilization or 8 weeks after implantation. In comparison to the embryo, the fetus has more recognizable external features, and a more complete set of developing internal organs. The entire process involves coordinated spatial and temporal changes in gene expression, cell growth and differentiation. A nearly identical process occurs in other species, especially among Chordates.