The pioneer work of Klein on unilaterally pregnant rabbits, rats and hamsters (Klein, 1933, 1935, 1938) established the role of the pregnant uterus in maintaining, independently of the foetuses, the histological structure and function of the corpora lutea of pregnancy. If the pregnant horn were removed from unilaterally pregnant animals in mid-pregnancy, degeneration of the corpora lutea followed, the progestational proliferation in the sterile horn broke down and a fresh ovulation took place.
From these, and from hypophysectomy experiments on the pregnant rat, a predominant role is often attributed to placental luteotrophins in animals where the corpora lutea persist for long periods or nearly till the end of pregnancy. This assumption has been tested by repeating Klein's experiments on the pregnant ferret.
The ferrets were from Mr Hammond's colony at the School of Agriculture, Cambridge. Normal pregnancies could be induced by