Karl Marx- Das Kapital, Mill’s Principles of Political Economy

When Marx was working on Das Kapital, Mill's Principles of Political Economy was the leading treatise of contemporary economics. In your opinion, what are two primary ways that Marx responded to Mill's work as an example of the classical economic model? (in other words, identify and explain two primary ways/ideas/concepts that Marx reacts to/extends upon/contradicts the classical model through Das Kapital?)