halacha - Surrogate mother

If a fetus has been formed in a mother and then deposited in a surrogate mother, who is considered the mother? In other words, does the mother's 'seed' make her the mother?

The story of Yosef and Dinah is well known. Yosef was originally in Leah and Dinah in Rochel. Because of their prayers they were 'changed' round and Yosef is considered Rochels child although made with Leah's 'seed'. Also, since it says one can always be sure who the mother is, that is why you use the mother's name, also proves that the surrogate mother is the one.