The step-by-step growth of the embryo follows, as it were, God’s “book” of instructions found in every cell. These instructions are thought to be found in a compound called deoxyribonucleic acid or DNA for short. Of the One who created DNA, it can be said: “In your book all its parts were down in writing.”

The mixing of DNA from both parents at conception is what produces the pattern that determines physical and mental tendencies. These make that child differ from all others. This occurs even before the “parts” are formed, just as the psalm continues, “as regards the days when they were formed and there was not yet one among them.” Those ‘parts down in writing’ are not just a “piece of tissue.” The potential personality traits of each embryo are there for the Creator to see, as if in his “book.”

Your eyes saw even the embryo of me, and in your book all its parts were down in writing, as regards the days when they were formed and there was not yet one among them.”​—Ps. 139:13-16, NW; compare Leeser’s translation.