<P> Human development is the process of growing to maturity . In biological terms, this entails growth from a one - celled zygote to an adult human being . </P> <P> Fertilization occurs when the sperm successfully enters the ovum's membrane . The genetical material of the sperm and egg that combine to form a single cell, called a zygote, and the germinal stage of prenatal development commences . The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation . The germinal stage is over at about 10 days of gestation . </P> <P> The zygote contains a full complement of genetic material and develops into the embryo . Briefly, embryonic developments have four stages: the morula stage, the bastula stage, the gastrula stage, and the neurula stage . Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions, called mitosis . A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother's uterus . This induces a decidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue . The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta . 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 8 weeks after fertilization . In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs . A nearly identical process occurs in other species . </P> <P> The following are some approximate age ranges for physical development stages: </P>

What are the four stages of human development