Complex sentence: The axiom schema of replacement was not part of Ernst Zermelo's 1908 axiomatisation of set theory (Z); its introduction by Adolf Fraenkel in 1922 is what makes modern set theory Zermelo- "Fraenkel" set theory (ZF).
Split into simpler sentences: The axiom schema of replacement was not part of Ernst Zermelo's 1908 axiomatisation of set theory (Z). Its introduction by Adolf Fraenkel in 1922 is what makes modern set theory Zermelo- "Fraenkel" set theory (ZF).