What I see are chemical gradients, such as in the early blastocyst stage. The morula stage is symmetrical and lacks a gradient. This might be as far as it can go. It needs a potential to set a gradient before it can go any farther.
In the human body, the primary chemical gradient appears to be between the nervous system and the blood supply. This is inferred from the exterior membrane potential of neurons with is positive/maximized, and the blood supply which is slightly alkaline or slight negative charge. The ratio of these (net potential in the context of ratio and quantity) helps maintain differentiation.
As long as the gradients are not damaged the configurational equilibria (team shape) associated with the DNA can develop normally. The configurational equilibria on the DNA (team shape) made easier by the way chromosomes are organized. These are also in the form of gradients. This is inferred based on aspects of the DNA remaining packed, while unpacking depends on chemical equilibrium.
We have an external gradient imposing external potentials, which impact the potential gradient at the membrane (membrane potential), which impacts the genetic gradient. This allows the genetic teams to be more than the sum of its parts. If we lose one gene the equilibrium DNA configuration redistributes in 3-D.
Keep in mind the gradient shown was inside the embryo, not an external influence. And this is not a chemical gradient per se, but a variation on the concentration of proteins that will turn on transcription factors inside the stem cells, to promote further differentiation. What you are seeing is programmed mutations taking place in a way that fast-forwards through the countless stages of evolution, yet implemented by forcing the transcription factors to unzip and express particular genes, even if these are only needed to build the banded regions where future generations of stem cells will "evolve".
The clip shows how nature exploits chemistry in ways that defy simplification, or the imposition of "reasoned" design. The complexity is traced to stages of development, from the earliest of notochords (flatworms) to vertebrate marine animals, amphibians, reptiles, birds, and tetrapods, which bridged the evolution from reptilian to mammalian forms.
Each stage of evolution introduces "tweaks" but preserves huge inherited chunks of chemical processing layers in embryonic development. This gives further incontrovertible proof that "design" is by natural selection, nothing more, as the refinements seen in embryogenesis encapsulate the successes of prior life forms, retain them, and reuse them
simply because they work and are handed down genetically.
As you see, once life forms were able to produce one "stripe" by this method, nature has arrived at 14 stripes by the time we get to the mosquito. And each of those stripes becomes a zone for further specialization to a body part, so if zone 10 were to represent the shoulder area, that is where the appendage for the front legs will further develop, from a bud.
The human embryo follows the exact game plan. As mentioned by billvon above, a thousand genes may be involved in the development of a particular organ. A particular gene has such a specific purpose, that the overall effect, to produce a particular organ or tissue, is only possible because hundreds or thousands of these interactions are carried out in sequence upon sequence, until each of the 200 or so different cell types (in the human body) are completely and finally differentiated out of the stem cells that work their magic during early development.
We talked about the development from zygote to blastocyst, which is the hollow sphere, and you saw the genesis of the embryonic mass, where the stem cells arise in the banded regions. What I would like to show you next is what happens as the embryo first starts budding and amassing primitive structures that are localized to each band, and yet able to maintain a connection, like a string of pearls, so the body plan remains continuous, yet able to differentiate locally.
This next clip begins where we left off, only here we see the early embryo from outside, as the striped zones begin to specialize further. You will see a flat substrate, which is the embryo itself. This shape proves we evolved from reptiles, because they needed more yolk to survive egg deployment on dry land. The space needed by the yolk required a flat embryo. Mammals don't require a yolk, but we inherited the flat structure, because nature had already designed it, and nature doesn't fix what aint broke. From this flat embryo a "primitive streak" develops. This is inherited from worms. The notochord they possess appears out of the streak, and will form two ridges. What happens next is a folding and curling that was worked out in early vertebrates to form the spinal tube and neural crest from which the vertebrae, spinal chord and brain will bud and develop. What you see playing out is a reenactment of evolution, in stage upon stage of formation and refinements added by mutation upon mutation. Yet these stages are stitched together seamlessly, so the transitions of 400 million years pass before your eyes in moments, or in a few weeks in the life of a human embryo. Notice that the embryonic disk differentiates into germ layers (ectoderm, endoderm, mesoderm) that each follow their own specialized tracks of development in this process, which is called gastrulation:
http://www.youtube.com/watch?feature=endscreen&NR=1&v=iHmBIJs77ZQ
and here you get a mapping between the germ layers and the systems, organs and tissues that will continue to differentiate within them:
http://www.youtube.com/watch?v=jblUFQIi_VY