Recently, in a brief communication, Coppola, Ball & Brown (1966) reported an incidence of 17% spontaneous deciduomata in pseudopregnant rats. One to three discrete zones of decidualization per uterus were observed. One of us, (M.C.S.), who has been concerned with the problem of decidualization for many years, has never observed as high an incidence of spontaneous decidualization. Because of the importance of the decidual response and decidual induction as a model system in the study of nidation (Shelesnyak, 1957) and of mammalian tissue growth and differentiation (Shelesnyak, 1962), it is essential to have a reliable index of the efficiency, or lack thereof, in induction of the decidual response under experimental conditions. Any degree of spontaneous decidualization would decrease the reliability of the experimental system by making unreliable such usual criteria as the weight of
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