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The placentation of Tupaia is of particular interest for two main reasons, first because of the suggested relationship of the Tree shrews (Tupaiidae) to the Lemuroidae—a relationship which has been much discussed and on which widely divergent views have been expressed—and second because Tupaia is the only mammal so far described in which the foetus is provided with a double (so called bidiscoidal) placenta of the entothelial chorial type.

The placental sites are easily recognizable in the virgin uterus as two localized areas of the endometrium, respectively dorsal and ventral, and characterized by the absence of uterine glands. On pregnancy supervening the endometrium of each site undergoes a marked decidual reaction forming the maternal component of (1) a temporary omphalopleural (yolk sac) placenta—the foetal component of which is furnished by the vascular omphalopleure—and of (2) the definitive allantoic placenta. The allantois growing down as a flattened lobe on either side and penetrating between the attached trophoblast of the vascular omphalopleure and its mesodermal and endothelial layers ensuring the replacement of the vitelline vessels of the vascular omphalopleure by the allantoic vessels and thus forming a relatively simple labyrinthine endothelio chorial placenta.