This paper reviews some of the more important literature relating to the morphological aspects of marsupial implantation in conjunction with hitherto unpublished observations. The data, although fragmentary, are derived from a wide variety of marsupial species and are presented in support of the thesis that fetal-maternal relationships in marsupials can most appropriately be divided into two phases. The first phase occupies up to two-thirds or more of pregnancy while the blastocyst undergoes expansion. The three primary germ layers (endoderm, mesoderm and ectoderm) are established and embryonic development proceeds at least as far as the pre-fetal or early fetal stage. Implantation is not known to occur in any marsupial during this period as an intact shell membrane completely separates fetal and maternal tissues. The second phase begins during the terminal third of gestation and
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