To him are due not only the names, but the idea, that the mammalian animals fall into three grades of ascending complexity of organisation: the reptile-like Prototheria, which lay large eggs, and which have many other reptilian characters; the Metatheria, or marsupial animals; the eutheria, or higher animals, which include all the common animals from the mole or rabbit up to man.
"Thomas Henry Huxley; A Sketch Of His Life And Work"
P. Chalmers Mitchell
In this way it seems to me we have a logical explanation of the fact that the corpora lutea in the Marsupial are not absorbed at parturition as in eutheria.
"Hormones and Heredity"
J. T. Cunningham