The picture at right, taken from work by Black and Gerhart (198?) demonstrates
one of the ways in which the normal rotation process can be overridden,
producing dramatic results. Here embryos were centrifuged after
rotation had begun, presumably producing a second relative displacement
of the cortex
and underlying material in the fertilized Xenopus egg. The second
displacement apparently is sufficient to produce a second region
capable of axis formation during subsequent development, resulting
in two headed-tadppoles.