"Early in embryonic development, the cells begin to specialize:  Some become liver cells; some become nerve cells; others become the transparent lens of the eye.  Every cell in the body carries the same genes.  (A gene, simply speaking, is a segment of DNA that encodes for a specific protein and therefore dictates a specific cell function.)  If all cells have identical genes, how can one cell be so different from another?  This question is currently the subject of extensive research.  Apparently, cells in various regions of the developing embryo are exposed to different chemical signals that channel the cells into specific pathways of development.”—Thank you, Elaine N. Marieb & Jon Mallat, for Human Anatomy, Second Edition, Benjamin/Cummings, 1997, pp.43-44