frog embryos that are a mirror image of their usual form have been created by American researchers using a protein that marks which part of the animal is going to be left side. The protein is the earliest step yet found in the pathway by which embryos differentiate one side from the other.

While vertebrates have bodies that are outwardly symmetrical, inside there are crucial differences. The two sides of the heart, for instance, perform different functions. The protein in question, vg1, belongs to a family of growth factors known as tgf-betas, several of which affect the development of left-right symmetry. When previous experimenters manipulated some of these tgf-betas, they were able to block development of the axis, so that asymmetric organs such as the heart developed randomly to left or right.

However, when graduate student Brian Hyatt injected a precursor of vg1 into certain cells of 16-celled embryos of the frog Xenopus laevis, the embryos switched left and right completely, so that the heart and other organs nearly always developed as a mirror image of their usual form. These embryos later "grew up" to be perfectly healthy frogs. This is the first time that experimenters have been able to completely reverse - instead of simply disrupt - left-right development (Cell , Vol 93, p 37).

The team also abolished the left-right axis on some frog embryos by tying these vertically very early. This produced a pair of conjoined twin embryos in which the left twin deve-loped a normal left-right axis while the right twin's asymmetric organs deve-loped randomly. Injecting vg1 into left-side cells of the right twin "rescued" the normal left-right axis, Hyatt discovered.

In other words, whichever side got vg1 eventually developed as the anatomic left.