SUMMARY The investigations performed on guinea-pig, rat, mouse and coypu (Myocastor coypus) placenta revealed some interesting aspects. In these species placenta is at the beginning of pregnancy of discoidal type and after the adhesion between the uterine mucosa and the chorial villies becomes of haemochorial type. The microscopic examiation of the placenta of guinea-pigs and coypus showed to be of a multilobated type while in the rat and the mouse the placenta looks like a half of one placentary lobe from guinea-pig or coypu. This observation could be an embryological proof concernig the phylogenetic evolution of rodents. Microscopic examination of rodents placenta showed that maternal lymphocytes cross the placenta and penetrate the embryonic trophoblast into the fetus that plead for a placentary immunity.
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