One of the mysteries of organic development is shape. How does a ball of cells change its shape into a chicken or a rhinoceros or a human?

For some years, researchers have tried to decipher the chemistry of the cellular events involved in the formation of limbs and other structures from an array of apparently similar cells.

The work has led to the search for morphogens, the chemicals that trigger shape. Now the first morphogen may have been found. In a report in the British journal Nature, Christina Thaller and Gregor Eichelle of the Harvard Medical School give evidence that a vitamin A derivative triggers the formation of limbs in embryonic chicks.

About 2 1/2 days after incubation begins, some cells form buds that will eventually become the wings or feet. Thaller and Eichelle found that a chemical called retinoic acid, derived from vitamin A, if applied to the area where limb-formation starts, can induce the formation of extra digits. Where three "fingers" would have formed, six were produced in a symmetrical pattern from "pinky" to "thumb" followed by their mirror image. The chemical triggers the formation according to how much of it is present -- each higher concentration brings out a new digit.

The finding suggests that a similar chemical trigger, perhaps even the same chemical, may be used by other species to make wings, hands, flippers, claws.

Organs of other shapes presumably also have chemical signals that coax cells to grow into hearts, brains and so on.