Complex sentence: The Swiss geometer Ludwig Schläfli pioneered this by defining "polyschemes", which mathematicians nowadays call polytopes; these are the analogues to polygons and polyhedra in spaces with more dimensions.
Split into simpler sentences: The Swiss geometer Ludwig Schläfli pioneered this by defining "polyschemes", which mathematicians nowadays call polytopes. These are the analogues to polygons and polyhedra in spaces with more dimensions.