An Introduction to the Human Growth..

Human Development is the process of growing to maturity. This entails growth from a one-celled called zygote to an adult human being.  Development begins with fertilization, the process by which the male gamete, the sperm cell and the female gamete, the egg, fuse to produce a zygote. In pregnancy the 3 stages are commonly referred as Z.E.F.  meaning Zygote, Embryo, Fetus. The beginning of pregnancy is the instant a sperm cell enters an ovum and forms a viable zygote. Recently, pregnancy is defined as beginning when a zygote becomes implanted in a woman's uterus. This occurs when the zygote then becomes embedded into the endometrium (lining of the uterus) where it forms a placenta, for the purpose of receiving essential nutrients through the uterus wall. The umbilical cord in an unborn child helps get the nutrients to the child and helps get rid of the waste from the child. Before the placenta is developed the blastocyst receives its nutrients from the yolk sac, which is contained within the blastocyst. The zygote undergoes rapid cell divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.