The key difference between Prototheria Metatheria and Eutheria is that Prototheria refers to egg-laying mammals while Metatheria refers to marsupials that give birth to partially developed young ones, and Eutheria refers to placental mammals that give birth to well-developed young ones.
Based on the major variations in reproductive systems, mammals can be categorized into three subclasses: Prototheria, Metatheria and Eutheria. Prototheria includes the most primitive mammals, which are egg-laying mammals, especially monotremes. Metatheria includes marsupials while Eutheria includes true placental mammals. In fact, Eutheria is the largest subclass of mammals.
