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If cell division continued in this fashion, then nine months later the hapless mother would give birth to a tumorous ball of literally astronomical proportions. But instead of endlessly dividing, the zygote's cells progressively take form. The first striking change is apparent four days after conception, when a 32-cell clump called the morula (which means "mulberry" in Latin) gives rise to two distinct layers wrapped around a fluid-filled core. Now known as a blastocyst, this spherical mass will proceed to burrow into the wall of the uterus. A short time later, the outer layer of cells will begin turning into the placenta and amniotic sac, while the inner layer will become the embryo.
The formation of the blastocyst signals the start of a sequence of changes that are as precisely choreographed as a ballet. At the end of Week One, the inner cell layer of the blastocyst balloons into two more layers. From the first layer, known as the endoderm, will come the cells that line the gastrointestinal tract. From the second, the ectoderm, will arise the neurons that make up the brain and spinal cord along with the epithelial cells that make up the skin. At the end of Week Two, the ectoderm spins off a thin line of cells known as the primitive streak, which forms a new cell layer called the mesoderm. From it will come the cells destined to make the heart, the lungs and all the other internal organs.
At this point, the embryo resembles a stack of Lilliputian pancakes--circular, flat and horizontal. But as the mesoderm forms, it interacts with cells in the ectoderm to trigger yet another transformation. Very soon these cells will roll up to become the neural tube, a rudimentary precursor of the spinal cord and brain. Already the embryo has a distinct cluster of cells at each end, one destined to become the mouth and the other the anus. The embryo, no larger at this point than a grain of rice, has determined the head-to-tail axis along which all its body parts will be arrayed.
How on earth does this little, barely animate cluster of cells "know" what to do? The answer is as simple as it is startling. A human embryo knows how to lay out its body axis in the same way that fruit-fly embryos know and C. elegans embryos and the embryos of myriad other creatures large and small know. In all cases, scientists have found, in charge of establishing this axis is a special set of genes, especially the so-called homeotic homeobox, or HOX, genes.
HOX genes were first discovered in fruit flies in the early 1980s when scientists noticed that their absence caused striking mutations. Heads, for example, grew feet instead of antennae, and thoraxes grew an extra pair of wings. HOX genes have been found in virtually every type of animal, and while their number varies--fruit flies have nine, humans have 39--they are invariably arrayed along chromosomes in the order along the body in which they are supposed to turn on.