“The [human] egg is one hundred times larger than the sperm it pairs with. Fortunately, the sperm doesn’t have to force its way in, but is welcomed like a long-lost if curiously diminutive friend. The sperm passes through an outer barrier called the zona pellucida and, all being well, fuses with the egg, which immediately activates a kind of electrical force field around itself to stop other sperm from getting through. The DNA from sperm and egg are combined into a new entity called a zygote. A new life has begun.
“Within a week [of conception] the zygote has produced ten or so cells known as pluripotent stem cells. These are the master cells of the body and one of the great miracles of biology. They determine the nature and organization of all the billions of cells that transform a little ball of possibility (known formally as a blastocyst) into a functioning and adorable little human (known as a baby). This moment of transition, when cells begin differentiating, is called gastrulation and has been described many times as the most important event of your life.” – Bill Bryson