Following Fraenkel's discovery, demonstrating that the rabbit corpora lutea are indispensable in pregnancy maintenance, Corner and Allen showed (1) in a series of now classic studies that luteectomy predictably terminates pregnancy, except when progesterone is provided as substitution therapy. Despite the clarity of these demonstrations and the accumulation of complementary data over 40 years (2, 3, 4), many investigators held the view that the studies in the rabbit model failed to establish a basic biological law of broad …