11.   At the same time, the embryos will be tissue-typed to find the best match with Zain.

12.   But embryos are incredibly resilient, he said.

13.   But in these cases the implanted embryos were not the result of cloning.

14.   Colborn gathered evidence that young embryos are vulnerable to even minute amounts of chemicals that mimic or counteract natural hormones.

15.   Crockin said the fact that an embryo is in a freezer vial rather than in a womb makes a great difference.

16.   Even closer at hand is a possible cancer cure, or a test that tells which embryos are prone to develop cancer prematurely.

17.   Even so, researchers hoping to publish work on nuclear transfer in humans may now have to come up with better evidence that embryos are healthy.

18.   Ian Wilmut of the Roslin Institute near Edinburgh agrees that is a possibility and says that human embryos could also be sensitive to this effect.

19.   In mammals, Behringer explains, male and female embryos are identical in the early stages of development.

20.   In such cases, the embryos are so old that they likely would not survive inside a womb.