If a pronoun can refer to more than one antecedent, revise the sentence to make the meaning clear.

What fell into the water—the car or the bridge? The revision makes the meaning clear by replacing the pronoun it with the bridge.

Reporting Kerry’s words directly, in quotation marks, eliminates the ambiguity.

If a pronoun and its antecedent are too far apart, you may need to replace the pronoun with the appropriate noun.

In the original, the pronoun it is too far away from the antecedent zygote in the first sentence, thus making the second sentence unclear to readers.