David, who penned those words, had no knowledge of cells and genes, being without even a microscope. But he correctly discerned that the development of his own body attested to advance planning. David may have had some knowledge of how embryos develop, so he could reason that each step must take place according to a preexisting design and timetable. In poetic language, he described this design as being "down in Writing" in God's "book." Building instructions for our body are "written" in the chemical structure of your personal DNA. Every time your cells divide--to make new cells or to replace old cells--your DNA passes those instructions on, thereby keeping you alive and maintaining your basic appearance. What an outstanding example of the power and wisdom of our heavenly Maker!
The picture is of my Great-grand-daughter Madyson. She was born on May 12, 2009.