That's 12(v). It's worded that up until the point that the stem cell line is created, you cannot withdraw, and the reason behind that comes from an old philosophical perspective in political philosophy having to do with law. You've mixed your labour with it, and therefore, in fact, you have some property rights in the stem cell. The stem cell didn't exist, but for you as the scientist doing a certain amount of work...and in fact, what was donated is no longer there to be retrieved. So what you gave me as an embryo, I can't give back to you. It doesn't exist anymore. That's part of the reason for saying that this would be a reasonable point at which it cannot be returned to.
The interesting thing, then, is in the context of not doing stem cell research, but donor gametes.... I've given you my eggs. At what point can I say no, give them back to me? I would say until you've taken them and transformed them into something else, until you've made them into an embryo, well, then you can say back to me, “Mrs. Baylis, I'd love to give you back your eggs, but I don't have them anymore. They've been made into an embryo. You can't have them back.”
The problem there is with the way it's worded. It says that you can withdraw up until the third party acknowledges in writing that the human reproductive material was designated for that use. I'm saying “was received”. That puts it just a little further down. I understand the problem, then, is who receives it. So it's not an uncomplicated issue.