Complex sentence: The first semantic theories designed to be compatible with transformational syntax were "interpretive"; that is, syntactic rules enumerated a set of well-formed sentences, each of which was assigned an "interpretation" by the rules of a separate semantic theory.
Split into simpler sentences: The first semantic theories designed to be compatible with transformational syntax were "interpretive". Syntactic rules enumerated a set of well-formed sentences, each of which was assigned an "interpretation" by the rules of a separate semantic theory.