Biological Development
From a biological standpoint, human development is a continuum, starting with the germ cells (ovum and spermatozoon), through fertilization, prenatal development, birth, 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. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions. A week after fertilization the embryo still has not grown in size, but hatches from its protein shell 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 embeded 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 either 8 weeks after fertilization or 8 weeks after implantation. 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, especially among Chordates.
Childbirth is the process in which the baby is born. Age is defined relative to this event in most cultures.
Read more about this topic: Human Development (biology)
Famous quotes containing the words biological and/or development:
“If the most significant characteristic of man is the complex of biological needs he shares with all members of his species, then the best lives for the writer to observe are those in which the role of natural necessity is clearest, namely, the lives of the very poor.”
—W.H. (Wystan Hugh)
“This was the Eastham famous of late years for its camp- meetings, held in a grove near by, to which thousands flock from all parts of the Bay. We conjectured that the reason for the perhaps unusual, if not unhealthful development of the religious sentiment here, was the fact that a large portion of the population are women whose husbands and sons are either abroad on the sea, or else drowned, and there is nobody but they and the ministers left behind.”
—Henry David Thoreau (1817–1862)