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Other substances such as serotonin and acetylcholine have also been shown to create morphogenetic fields during embryogenesis. We'll discuss one more case of differentiation under the influence of a chemical substance. It illustrates the variable effect of chemical oscillations that act first on one partition or 'resonating space', and in following stages find that the resonating space has split up in parts, so that the oscillation produces 'partials'. I summarize a description by S. A. Newman and H. L. Frisch (1984) on the embryonic formation of limbs.
The limbs of vertebrate species show special adaptations. There are limbs for walking, elongated limbs for swinging through trees, fins and flippers for swimming and paddling, wings for gliding and flapping. Different though they may be, they are variations upon a common theme. Extending outward from the body, an increasing number of bones are found within a sheath of skin. The human arm has one humerus, two bones in the forearm (the radius and ulna), two rows of four in each wrist and five jointed chains of bones for each hand. The leg and foot show an analogous arrangement; so do the body-parts in other species that are used for motion.
The phenomenon of distal multiplication is explained by a peculiar 'dance of the cells' that is executed during morphogenesis of those limbs. In the mesenchyme (the primary substance of the beginning limb bud) some cells close to the body wall are transformed into cartilage: they aggregate and form tight clusters. Neighbouring cells follow, and together they change from individual cells, spread out over a given space, into a close-knit tissue of cartilage cells in the middle of the space where later the bone will form. They aggregate under the influence of a chemical signal, fibronectin, that is distributed unevenly in the mesenchymal area. , The agglomeration and subsequent changing of the cells occurs where its concentration is highest It is this concentration of cells that is dancing to the music of resonating chemical waves. In a closed space, where wave propagation is thrown back on itself, there will be a standing wave pattern, with nodes where the concentration of the proteïn is highest. Resonance will take place in a fundamental tone mode (no harmonics) or in an oscillating-mode with one or two harmonics (that is: peaks and valleys in the concentrations of fibronectin), when the resonating space becomes smaller. As the limb formation progresses, the the partition keeps narrowing: the number of nodes will multiply, and so will the number of bones. In the view of these authors the differentiation of parts of vertebral limbs (time window of epigenesis) is analogous to the production of overtones in a flute or a violin (time window of audio-frequencies).
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