Close apposition of the mucous surfaces—the attachment reaction—occurs on implantation in the mouse (Potts, 1966, 1968; Nilsson, 1967; Reinius, 1967; Potts & Psychoyos, 1967b) and the rat (Mayer, Nilsson & Reinius, 1967; Nilsson, 1967; Potts & Psychoyos, 1967a). Since both these species have an eccentric implantation, it seemed possible that the occurrence of the attachment reaction might be correlated with the type of implantation. The ultrastructure of the uterine surface epithelium was therefore examined at preimplantation and implantation in animals representative for different modes of implantation —eccentric (the hamster), central (the rabbit and the mink) and interstitial (the guinea-pig).
The animals were bred under standardized conditions. Mating of the animals was verified in the hamster and guinea-pig by the presence of vaginal spermatozoa (Day 1 of