Reexamination of presomite human and rhesus monkey embryos in the Carnegie Collection provides no evidence to corroborate the hypothesis that the trophoblast is the source of all extraembryonic tissues in these embryos. Instead, the present study indicates that the developmental pattern of the yolk sac and extraembryonic mesoderm is homologous to that in other eutharian mammals. The primary yolk sac of 10- to 11-day human blastocysts is partially filled with a meshwork of extraembryonic endoderm, whereas such a meshwork is absent in the rhesus monkey. It is suggested that this endodermal meshwork develops as the result of interstitial implantation in the human embryo. A small secondary yolk sac develops in 12- to 13-day human and macaque embryos as the result of pinching off of a portion of the larger primary yolk sac. Development of a secondary yolk sac in higher primates appears to be related causally to differential rates of expansion of the blastocyst and primary yolk sac within the simplex uterus. The caudal margin of the primitive streak develops precociously in 12- to 14-day human and macaque embryos, and this appears to be the source of all the extraembryonic mesoderm of the chorion, chorionic villi, and body stalk. It is suggested that the peripheral spread of extraembryonic mesoderm plays in inductive role in the development of chorionic villi, similar to other types of epithelial-mesenchymal inductive interactions. In contrast to previous hypotheses, the human and macaque trophoblasts appear to give rise only to additional trophoblast.

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Selected characters of the cranioskeletal, dental, neuroanatomical, reproductive, lymphoid, and developmental systems that are known for most or all extant marsupial families are reviewed and analyzed for their abilities to corroborate higher-level hypotheses of suprafamilial relationships among extant marsupials. In addition, relatively conservative nucleotides from the mitochondrial 12S rDNA gene (obtained from the recent study by Springer et al., J. Mammal. Evol. 2, 85-115, 1994) were incorporated into the same character analysis. The ancestral morphotype for each marsupial family and each character was reconstructed, using the reconstructed eutherian and therian morphotypes for outgroup comparisons. In addition, ontogenetic data, stratigraphic position of fossils, and form-functional considerations were used, whenever feasible, to assess character state polarity of anatomical traits. Despite missing data from some families and many genera, this preliminary and modified "total evidence" approach helps to identify several well corroborated higher taxa, including Diprotodontia, Vombatiformes, and Macropodoidea, and it provides modest support for Ameridelphia, Australidelphia, and Syndactyla. An important conclusion is that no single data set is capable of resolving all suprafamilial relationships among marsupials. Suggestions are also presented for future multidisciplinary approaches to help resolve several polychotomies that have remained resistant to phylogenetic analyses of single data sets.

Re-examination of early rhesus monkey and human embryos in the collection of the Carnegie Institution of Washington suggests that the mechanism of amniogenesis in both is basically similar to that of the hedgehog and vespertilionid bats. A primordial amniotic cavity develops by cavitation within the embryonic mass of 10-day rhesus monkey, and 7-day human, blastocysts. This primordial cavity has no relationship initially with the overlying trophoblast, contrary to earlier reports. Subsequently, there is a thinning and peripheral spreading of the epiblastic roof of the primordial cavity, resulting in partial opening of the roof and formation of a slightly cupped embryonic disc. The resulting space is not homologous with the primordial amniotic cavity; instead, it is a transitory tropho-epiblastic cavity. The definitive amniotic epithelium forms by the upfolding and mitotic proliferation of the margins of the epiblastic disc; this process is completed in 11-day rhesus, and 9-day human, blastocysts. Amniogenesis by cavitation is associated with the persistence of polar trophoblast following implantation, and it is suggested that this cavitation process may be essential for providing a free epithelial surface for the morphogenetic movement of epiblastic cells during subsequent formation of the primitive streak.