Complex sentence: It was several more years before a functional analysis of the vertebrate "slit" and "robo" mutants was performed, and demonstrated that Slit-Robo signaling regulates commissural axon guidance in vertebrates as well.
Split into simpler sentences: It was several more years before a functional analysis of the vertebrate "slit" and "robo" mutants was performed. After the analysis it was clear that Slit-Robo signaling regulates commissural axon guidance in vertebrates as well.