Derivation of Cells
Human beings begin life as a single, newly fertilized cell. Like every cell that contains a nucleus, the fertilized cell holds all the instructions for its growth and development. The characteristics common to all living cells include the ability to reproduce, exchange gases, move, react to external stimuli, and create or utilize energy to perform their tasks.
Shortly after the ovum or egg is fertilized, it divides to form two cells. These two cells then divide to form a total of four, which again divide to form eight and continues on. This group of cells continues dividing; after nine days it attaches to the wall of the uterus and becomes an [glossary term:] embryo.
About two weeks after conception, the cells of the embryo continue to divide, changing their shape and structure. This process is known as differentiation. The cells arrange into distinct layers called germ layers: an outer ectoderm and inner endoderm (entoderm). A third embryonic layer, the mesoderm, develops between the ectoderm and the endoderm. All the organs of the body develop or differentiate in an orderly fashion from these three primary germ layers.