“…23 The classic example, of course, is the work of Long and Evans in 1922, whose experiments revealed that the ovaries of immature female rats (of their now famous Long-Evans strain obtained by crossing wild Norwegian grays with Wistar albinos) could be induced to undergo maturation when transplanted into adult females. Using the same assay, in 1925, Frank and Gustavson, and later in the year, Frank, Kingery, and Gustavson, 27,28 while investigating whether the normal waiting period to puberty in Long and Evans' rats was due to "restraining influences…at work" or due to the "absence of the female sex hormone", postulated that lipoid extracts of ovarian follicles, corpora lutea, and the placenta, which presumably all contained the same hormone, were capable of inducing vaginal opening and hastening the first ovulation in immature rats. This discovery became the subsequent basis of the Allen-Doisy test, in which a substance suspected of containing estrogens is injected into an ovariectomized mouse, and vaginal smears examined for the presence 5.…”