Complex sentence: The first had general aspects of relativistic quantum mechanics and relativistic quantum field theory, leading onto quantum electrodynamics, the second continued on quantum electrodynamics and what was then known about the strong and weak interactions.
Split into simpler sentences: The first had general aspects of relativistic quantum mechanics and relativistic quantum field theory, leading onto quantum electrodynamics. The second continued on with quantum electrodynamics and what was then known about the strong and weak interactions.