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The fetal membranes and unusual giant cell placenta of the jerboa (Jaculus) and jumping mouse (Zapus).

The development of the fetal membranes of the North African jerboa (Jaculus) and the North American jumping mouse (Zapus) were examined by light microscopy, and the interhemal membrane of Zapus was studied by electron microscopy. Early stages of Jaculus show implantation to be antimesometrial with the embryonic disc oriented mesometrially. Amniogenesis is by folding and there is a distinct epamnionic cavity. Yolk sac inversion is complete, but the disappearance of the parietal segment occurs relatively late, during the early fetal period. Those early stages of Zapus which were available indicated a close similarity to Jaculus. Both Jaculus and Zapus are unusual in the manner of development and composition of the chorio-allantoic placenta. Development of cytotrophoblast and syncytiotrophoblast is arrested early and these layers eventually disappear. Trophoblastic giant cells migrate into the allantoic mesenchyme to form the maternal blood channels of the labyrinth. In Jaculus, a complex interdigitation exists between yolk sac villi and arterial channels bringing maternal blood to the labyrinth. This relationship is much less intimate and extensive in Zapus. Electron microscopic examination of the interhemal membrane of Zapus shows it to be hemomonochorial, its only trophoblastic element being the giant cell cytoplasm. Thus, the Zapus placenta is to date one of very few hemochorial placentas known to lack trophoblastic syncytium in its interhemal membrane. Comparison of the total development and structure of the fetal membranes in these two genera with that of other rodents indicates that the membranes of the Dipodoidea, like the Geomyoidea, are intermediate in type between those of the Sciuromorpha and Myomorpha.