Facing off below, identical tadpole twins—generated by cutting an embryo in two equal halves—are indistinguishable from the typical tadpole above.

It was in 1903 that biologist Hans Spemann— using a loop of his baby daughter's hair and a newt egg—revealed the startling embryological mystery that would persist for more than a century. Testing the adaptability of an embryo, Spemann deftly lassoed the egg with the hair and constricted it so that all nuclear divisions occurred only on one side. Eventually, a nucleus would escape through the constriction to the other side and nuclear divisions would begin there as well. At this point, he would tighten the lasso to completely separate the two sides. To his amazement, both halves developed into identical, perfectly normal, half-sized embryos. This embryonic fail-safe machinery doesn't only reside in amphibians, though. Identical twins often result when such egg splitting occurs in humans.

Now, HHMI investigator Edward M. De Robertis and graduate student Bruno Reversade have made an important advance in revealing the molecular mechanism underlying this remarkable resilience—namely, the "morphogenetic fields" that govern embryonic development. Such fields are gradients of regulatory proteins that guide differentiation of embryonic cells and organize the embryo's overall shape. Although researchers have long known that such fields exist, little was known about the molecular basis of their function.

Photo: Courtesy of Hiroki Kuroda/De Robertis Lab

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