Placentation in the colugos Cynocephalus volans and Galeopterus variegatus (Dermoptera) and the transition from labyrinthine to villous placentation in primates.

Introduction: Phylogenetics and genomics place colugos as the sister group to primates. Therefore their placentation is of interest in an evolutionary perspective. Previous accounts are fragmentary, not readily accessible and sometimes contradictory.

Methods: We have examined archival material covering the early development of fetal membranes and placenta, the fate of the yolk sac and definitive placentation.

Results: Initially the trophoblast extended over a rather broad but shallow area, enclosing maternal blood spaces. After expansion of the exocoelom it became covered by somatic mesoderm. The mature chorioallantoic placenta was haemochorial and characterized by a labyrinth with markedly dilated maternal blood spaces. Blood vessels appeared in the splanchnopleure early in development and later extended to the yolk sac, but we found no evidence of a choriovitelline placenta at any stage of gestation. There was, however, an extensive paraplacenta.

Conclusions: A choriovitelline placenta is not formed early in gestation nor is it present at term. Early in development invasive trophoblast spreads laterally to form a trophoblastic plate. We found evidence to support the idea that the colugo placenta is intermediate between the labyrinthine placenta of rodents and the trabecular type of Neotropical primates.