The suppression of oestrus in the mouse during placental retention (Newtoil, '35) implies a direct o r indirect relationship between placenta and ovary. In man, monkeys and some other animals in which the ovaries are unnecessary for continued gestation after the early stages of pregnancy, a case can be made out for a straightforward depression of the ovaries, particularly as these may show atrophic changes. N o such simple relationship can be postulated for the rat and mouse, since the ovaries of these animals cannot he removed without terminating pregnancy.Iilein ( '35) has shown very clearly that iii the rat with retained placentae mucification of the vaginal epithelium is dependent on the presence of the placentae, but cannot be maintained in the absence of the ovaries. Froin this and previous work on the endometrium of the rabbit ('33)' Klein suggests a stimulating action of the placenta on the corpus luteum. However, there have been indications that the placenta itself may also produce progesterone, o r exert an endocrine influence independently of the o17ary. Thus Selye, Collip and Thomson ( ' 3 5 ) on the grounds of endometrial changes and maintenance of the mammary glands in rats, and Haterius ( '36), who succeeded in maintaining gestation in oophorectomized rats from which all except one foetus, but

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