In Tadarida, the diffuse endotheliodichorial placenta is established over the parietal area of the uterus and is prominent, for an extended period of time during midgestation, both prior to and during the development of the definitive discoidal structure. A transitory yolk sac placenta precedes the establishment of the diffuse placenta. The tissues of the yolk sac are displaced from the trophoblast through collapse, and by the advance of the allantoic mesoderm. Allantoic vessels infiltrate the cords of cellular trophoblast that extend into the syncytium that, in turn, has penetrated deeply into the maternal endometrium. The syncytium progressively phagocytizes the maternal tissues, leaving the superficial maternal blood vessels and attendant pericytes intact, but destroys many of the deeper maternal vessels. Several morphological changes occur in the pericytes during the period that the superficial maternal capillaries retain their integrity, suggesting that they play an important role in maintaining these vessels which are eventually destroyed also.
The cytotrophoblast is the most undifferentiated cell type throughout gestation. The syncytium, on the other hand, contains a large array of organelles that vary in concentration from one area to another within this tissue. Particular attention was paid to cytoplasmic structures potentially related to transport phenomena. Among those described are (1) coated vesicles and tubules in the syncytium at its interface with the cellular trophoblast, (2) microvilli extending from the cytotrophoblast into the syncytium some of which contained lines of pinocytotic vesicles, (3) coated vesicles along the plasma membrane of the syncytium toward the maternal tissues, and (4) pinocytotic vesicles associated with the endothelium of both the allantoic and maternal blood vessels.